Piston Engine Systems
Starting and Ignition System
Types of starting system in piston engines are
Direct hand cranking
Direct cranking, either hand or electrical
Combination hand and electric inertia.
First two may be described as direct cranking systems and second two may be referred to as inertia type cranking
Direct Cranking System
Direct Hand Cranking Starter
The direct hand-cranking starter is sometimes described as a hand-turning gear-type starter. It consists of a worm-gear assembly that operates an automatic engaging and disengaging mechanism through an adjustable-torque overload - release clutch. It has an extension shaft that may be either flexible or rigid, depending on the design. To prevent the transmission of any reverse motion to the crank handle in case the engine “kicks” backward while it is being cranked, a ratchet device is fitted on the hand crankshaft.
This type of starter can be used with a gear ratio of 6:1 for any engine rated at 250 hp or less. It has a comparatively low weight, and it is simple to operate and maintain. It was extensively used on early airplanes which had low horsepower engines and no source of electric power for starting. On seaplanes, where it was very difficult to start the engine by swinging the propeller, it was especially popular. However, it has been entirely supplanted by more efficient design.
Direct Cranking Electric Starter
When the direct-cranking method is used, there is no preliminary storing of energy in the flywheel as there is in the case of the inertia-type starters. The starter of the direct type, when electrically energized, provides instant and continuous cranking. The starter fundamentally consists of an electric motor, reduction gears, and an automatic engaging and disengaging mechanism, which is operated through an adjustable torque overload-release clutch. The engine is therefore cranked directly by the starter.
The motor torque is transmitted through the reduction gears to the adjustable torque overload-release clutch, which actuates a helically splined shaft. This, in turn, moves the starter jaw outward, along its axis, and engages the enginecranking jaw. Then, when the starter jaw is engaged, cranking starts.
When the engine starts to fire, the starter automatically disengages. If the engine stops, the starter automatically engages again if the current continues to energize the motor.
The automatic engaging and disengaging mechanism operates through the adjustable torque overload-release clutch, which is a multiple-disk clutch under adjustable spring pressure. When the unit is assembled, the clutch is set for a predetermined torque valve. The disks in the clutch slip and absorb the shock caused by the engagement of the starter dogs. They also slip if the engine kicks backward. Since the engagement of the starter dog is automatic, the starter disengages when the engine speed exceeds the starter speed.
The most prevalent type of starter used for light and medium engines is a series electric motor with an engaging mechanism. In all cases, the gear arrangement is such that there is a high gear ratio between the starter motor and the engine. That is, the starter motor turns many times the rpm of the engine.
There are two types of inertia starters :
The hand-cranking type, commonly called the hand inertia starter, in which the flywheel is accelerated by hand only ; and
The electric type, commonly called the combination inertia starter and sometimes referred to as the combination of hand and electric inertia starter. In which the flywheel is accelerated by either a hand crank or an electric motor.
Principles Governing The Operation Of The Inertia Starter.
Newton’s first law states : Every body continues in its state of rest or uniform motion in a straight line unless it is compelled to change that state by some external force. This is also known as a statement of the property of inertia.
The cranking ability of an inertia starter for airplane depends on the amount of energy stored in a rapidly rotating flywheel. The energy is stored in the flywheel slowly during the energizing process, and then it is used very quickly to crank the engine rapidly, thus obtaining from the rotating flywheel a large amount of power in a very short time.
Under ordinary conditions, the energy obtained from the flywheel is great enough to rotate the engine crankshaft three or four times at a speed of 80 to 100 rpm. In this manner, the inertia starter is used to obtain the starting torque needed to overcome the resistance imposed upon the cranking mechanism of the engine by reason of its heavy and complicated construction.
The speed at which the engine crankshaft is rotated may be less than the coming-in speed for the magneto, which is the minimum crankshaft speed at which the magneto will function satisfactorily. Therefore, if the engine uses magnetos for ignition, as practically all modern engines do, an ignition booster of some type must be provided. It is usually installed on or near the engine and operated while the inertia starter is cranking the engine.
Hand Inertia Starter
In using the hand inertia starter, when a hand crank is placed in the crank socket and the crank rotated, a gear relationship between the crank and the flywheel makes it possible for a single turn of the hand crank to cause the flywheel to turn many times. For example, one revolution of the hand crank may cause the flywheel to revolve 100 or more times, depending on the make, model, etc., of the starter. The speed of all movable parts is gradually increased with each revolution of the hand crank, and most of the energy imparted to the crank is stored in the rapidly rotating wheel in the form of kinetic energy.
One type of clutch consists of one set of disks fastened to the shaft and another set of disks, made of a different kind of metal, fastened to the barrel. The disks are pressed together by springs. The retaining ring compresses the springs and can be adjusted to set the value of the slipping torque.
This feature is important because the normal operation of the clutch is to slip momentarily after the starter and engine jaws are meshed. During the process of slipping, a torque is exerted on the crankshaft until the initial resistance of the engine is overcome and the clutch is again able to hold. The maximum holding torque is called the break away. This break away and the slipping torque depend on the size of the engine being cranked.
Combination Hand And Electric Inertia Starters
A combination hand and electric inertia starter may consist of a hand inertia starter with an electric motor attached, and the gear and clutch arrangement may be like that of the hand inertia starter.
The flywheel may be accelerated by either a hand crank or the electric motor. When the starter is energized by hand cranking, the motor is mechanically disconnected and no longer operates. When the motor is operated, a movable jaw on a helically splined shaft engages the motor directly to the inertia starter flywheel in one type of inertia starter called a jaw = type starter motor-engaging mechanism.
On some starters of this general type, the starter jaw tends to remain at rest when the motor armature starts to rotate, but as the shaft turns, the jaw moves forward along the splined shaft until it engages the flywheel jaw. Ordinarily, there is no trouble with this type of mechanism, but if the jaw binds on the shaft, it will not engage the flywheel, and thus the motor races. When the engine fails to start, the operator must not continue to attempt cranking otherwise, the teeth may be stripped on either the flywheel for the motor jaw or both. The correct procedure is to wait until the starter flywheel comes to rest before energizing the motor in a second attempt to crank. This avoids both the racing of the motor and the stripping of the teeth.
Electric Motors For Inertia Starters
The typical electric motor used on inertia starter are series motors operated on 12 or 24 V D.C. and with a low resistance winding. It was a low resistance, because there is a great amount of current flow in order to provide a powerful starting torque. When the motor gain speed, the induced electromotive force (emf) in a reverse direction causes the smaller amount of current to flow i.e. a counter emf is established when the motor gain speed.
An inertia starter motor is never operated at full voltage unless there is a load imposed upon it. If there is no load, and if the motor has a small amount of internal friction, it will race and the armature may fly apart (“burst”) because of the centrifugal stresses.
A simple series motor field coils are connected in series with the armature. Since all the current used by the motor must flow through both the field and the armature it is apparent that the flux of both the armature and the field will be strong.
The greatest flow of current through the motor will take place when the motor is being started; hence, the starting torque is high. Series-wound motors are used wherever the load is continually applied to the motor and is heavy when the motor first starts. In addition to starting motors, motors used to operate landing gear, cowl flaps, and similar equipment are of this type.
Ignition System for Piston Engines
During ignition event of a piston engine, the electric spark jumps between the electrodes (points) of a spark plug that is installed in the cylinder head or combustion chamber of the engine cylinder. The ignition system furnishes sparks periodically to each cylinder at a certain position of piston and valve travel.
Essential Parts Of An Ignition System
The essential parts of an ignition system for a reciprocating engine are a source of high voltage, a timing device to cause the high-voltage source to function at the set position of piston travel, a distributing mechanism to route the high voltage to the various cylinders in the correct sequence, spark plugs to carry the high voltage into the cylinders of the engine and ignite the fuel air mixture, control switches, and the necessary wiring. The source of the high voltage may be either a magneto driven by the engine or an induction coil connected to a battery or a generator.
All parts of the aircraft ignition system are enclosed in either flexible or rigid metal covering called shielding. This metal covering “receives” and “grounds out” radiation from the ignition system, which would otherwise cause interference (noise) in the radio receiving equipment in aircraft.
Magneto ignition is superior to battery ignition because it produces a hotter spark at high engine speeds and it is a self contained unit, not dependent on any external source of electric energy. Magneto produces electric current in pulsations of high voltage for purpose of ignition. When an aircraft engine is started, the engine turns over too slowly to permit the magneto to operate; hence, it is necessary to use a booster coil, vibrating interrupter (induction vibrator), or impulse coupling for ignition during starting.
When two magnetos fire at the same or approximately the same time through two sets of spark plugs, this is known as double or dual, magneto ignition system. The principle advantage of dual magneto ignition are as follows :-
If one magneto or any part of one magneto system fails to operate, the other magneto system will furnish ignition until the disabled system functions again.
Two sparks, igniting the fuel-air mixture in each cylinder simultaneously at two different places, give a more complete and quick combustion than a single spark; hence, the power of the engine is increased.
On radial engines, it has been a standard practice to use the right-hand magneto for the front set of spark plugs and the left-hand magneto for the rear set of spark plugs.
Dual-ignition spark plugs may be set to fire at the same instant (synchronized) or at slightly different intervals (staggered). When staggered ignition is used, each of the two sparks occurs at a different time. The spark plug on the exhaust side of the cylinder always fires first because the slower rate of burning of the expanded and diluted fuel-air mixture at this point in the cylinder makes it desirable to have an advance in the ignition timing.
Polarity or Direction of Spark
Fundamentally the magneto is a special form of AC generator, modified to enable it to deliver the high voltage required for ignition purposes. The high rate of change of flux linkages are responsible for the high voltage which produces the strong spark. Flux change are in downward and upward direction, alternating in direction at each opening of the contacts.
Since the direction of an induced current depends on the direction of the flux change which produced it, the sparks produced by the magneto are of alternate polarity, that is, they jump one way and then the other.
High Tension Ignition System
One end of the primary winding is grounded to the magneto, and the other and is connected to the insulated contact point. The other contact point is grounded. The capacitor is connected across the contact points.
The ground terminal on the magneto is electrically connected to the insulated contact point. A wire called the P lead connect the ground terminal on each magneto with the switch. When the switch is in the OFF position, this wire provides a direct path to the ground for the primary current, that is, the breaker points are short-circuited. Therefore, when the contact points open, the primary current is not interrupted, thus preventing the production of high voltage in the secondary winding.
One end of the secondary winding is grounded to the magneto, and the other and terminates at the high-tension insert on the coil. The high-tension current produced in the secondary winding is then conducted to the central insert of the distributor finger and across a small air gap to the electrodes of the distribution block. High-tension cables in the distributor block then carry it to the spark plugs where the discharge occurs in the engine cylinder.
The distributor finger is secured to the large distributor gear which is driven by a smaller gear located on the drive shaft of the drive shaft of the rotating magnet. The ratio between these gears is always such that the distributor finger is driven at one-half engine-crankshaft speed. The ratio of the gears ensures the proper distribution of the high tension current to the spark plugs in accordance with the firing order of the particular engine.
In general, the distributor rotor of the typical aircraft magneto is a device that distributes the high-voltage current to the various connections of the distributor block. This rotor may be in the form of a finger, disk, drum, or other shape, depending on the judgement of the magneto manufacturer. In addition, the distributor rotor may be designed with either One or two distributing electrodes. When there are two distributing electrodes, the leading electrode, which obtains high voltage from the magneto secondary, makes its connection with the secondary through the shaft of the rotor, while the trailing electrode obtains a high-tension voltage from the booster by means of a collector ring mounted either on the stationary distributor block or on the rotor itself.
Distributor with the trailing finger are used on older aircraft. The modern engines utilizes booster magnetos or high tension booster coil to provide a strong spark when starting the engine.
Magneto sparking order
Almost all piston-type aircraft engines operate on the four-stroke five-event-cycle principle. For this reason, the number of sparks required for each complete revolution of the engine is equal to one-half the number of cylinders in the engine. The number of sparks produced by each revolution of the rotating magnet is equal to the number of its poles.
Therefore the ratio of the speed at which the rotating magnet is driven to the speed of the engine crankshaft is always one half the number of cylinders on the engine divided by the number of poles on the rotating magnet.
The numbers on the distributor block show the magneto sparking order and not the firing order of the engine. The distributor-block position marked 1 is connected to no 1 cylinder and no 2 to no 2 cylinder and so on.
Some distributor blocks or housing are not numbered for all high tension leads. In these cases the lead socket for the no. 1 cylinder is marked and the others follow in order according to direction of rotation.
Coming in speed of magneto
To produce sparks, the rotating magnet must be turned at or above a specified number of revolutions per minute, at which speed the rate of change in flux linkages is sufficiently high to induce the required primary current and the resultant high-tension output. This speed is known as the coming-in speed of the magneto; it varies for different types of magnetos but averages about 100 to 200 rpm.
Harness assembly consists of flexible shielding from the magneto housing to the rigid manifold that is suitably installed on the crankcase of the engine and flexible shielded leads from the manifold to the spark plugs. Thus the complete system is shielded to prevent the emanation of electromagnetic waves which would cause radio interference.
In a system of this type, the lower extremities of the manifold must be provided with drain holes to prevent the accumulation of moisture. In some systems, the manifold is completely filled with a plastic insulating material after the ignition cables are installed. This seals the cables completely away from any moisture.
Ignition harnesses for opposed engines consist of individual high-tension leads connected to the distributor plate or cap and routed to each spark plug in proper order.
Ignition switch and the primary circuits
The usual electric switch is closed when it is turned ON. The magneto ignition switch is closed when it is turned OFF.
This is because the purpose of the switch is to short-circuit the breaker points of the magneto and prevent collapse of the primary circuit required for production of a spark.
The ignition switches for modern aircraft have the appearance of automobile starter switches mounted on the dash board. The aircraft switch is operated by a key and has positions for OFF, RIGHT, BOTH and START. The switch has a connection for battery power which is used in the START position to actuate the starter contactor or relay. In some cases, the start-ignition switch also includes the master power switch for the aircraft.
Types of Magneto
Types of magnetos are
low tension, high tension
rotating-magnet or inductor-rotor,
single or double and
base-mounted or flange-mounted.
Low tension and high tension magnetos
A low-tension magneto delivers current at a low voltage by means of the rotation of an armature, wound with only one coil, in the field of a permanent magnet. Its low-voltage current must be transformed into a high tension (high-voltage current by means of a transformer.
A high tension magneto delivers a high voltage and has both a primary winding and a secondary winding. An outside induction coil is not needed because the double winding accomplishes the same purpose. The low voltage generated in the primary winding induces a high-voltage current in the secondary winding when the primary circuit is broken.
Rotating magnetos and inductor rotor magnetos
In a magneto of the rotating-magnet type, the primary and secondary winding are wound upon the same iron core. This core is mounted between two poles, or inductor, which extend to “shoes” on each side of the rotating magnet.
The rotating magnet is usually made with four poles, which are arranged alternately north and south in polarity. The inductor-rotor type of magneto has a stationary coil (armature) just as the rotating-magnet type does. The difference lies in the method of inducting a magnetic flux in the core of the coil. The inductor-rotor magneto has a stationary magnet or magnets. As the rotor of the magneto turns, the flux from the magnets is carried through the segments of the rotor to the pole shoes and poles, first in one direction and then in the other.
Single and double type magnetos
Two single type magnetos are commonly used on piston type engines. The single-type magneto is just what its name implies-one magneto.
The double-type magneto is generally used on different models of several types of engines, when made for radial engines, it is essentially the same as the magneto made for in-line engines except that two compensated cams are employed.
The double type magneto is essentially two magnetos having one rotating magnet common to both. It contains two sets of breaker points, and the high voltage is distributed either by two distributors mounted elsewhere on the engine or by distributors forming part of the magneto proper.
Base mounted and flange mounted magnetos
A base-mounted magneto is attached to a mounting bracket on the engine by means of cap screws, which pass through holes in the bracket and inter tapped holes in the base of the magneto.
A flange-mounted magneto is attached to the engine by means of a flange on the end of the magneto. The mounting holes in the flange are not circular; instead, they are slots that permit a slight adjustment, by rotation, in timing the magneto with the engine.
The single-type magneto may be either base-mounted or flange-mounted. The double-type magneto is always flange mounted.
It is impossible under certain conditions to rotate the engine crankshaft fast enough to produce the coming-in speed of the magneto, a source of external high-tension current is required for starting purposes. The various devices used for this purpose are called ignition boosters.
An ignition booster may be in the form of a booster magneto, a high-tension coil to which primary current is supplied from a battery, or a vibrator which supplies in intermittent direct current from the battery directly to the primary of the magneto. Another device used for increasing the high-tension voltage of the magneto for starting is called a impulse coupling. It gives a momentary high rotational speed to the rotor of the magneto during starting.
A booster coil is a small induction coil. Its function is to provide a shower of sparks to the spark plugs until the magneto fires properly. It is usually connected to the starter switch. When the engine has started the booster coil and the starter are no longer required; hence they can be turned off together.
When voltage from a battery is applied to the booster coil, magnetism is developed in the core until the magnetic force on the soft-iron armature mounted on the vibrator overcomes the spring tension and attracts the armature toward the core. When the armature moves toward the core, the contact points and the primary circuit are opened.
This demagnetizes the core and permits the spring again to close the contact points and complete the circuit. The armature vibrates back and forth rapidly, making and breaking the primary circuit as long as the voltage from the battery is applied to the booster coil.
Booster coils were used on older aircraft. Most of the modern aircraft employ the induction vibrator or an impulse coupling.
This circuit applies to one engine only, but it is obvious that a similar circuit would be used with each engine of a multi engine airplane. The induction vibrator is energized from the same circuit which energizes the starting solenoid. It is thus energized only during the time that the engines are being started.
One advantage of the induction vibrator is that it reduces the tendency of the magneto to “flash over” at high altitudes, since the booster finger can be eliminated. The function of this induction vibrator is to supply interrupted low voltage for the magneto primary coil, which induces a sufficiently high voltage in the secondary for starting.
The vibrator sends an interrupted battery current through the primary winding of the regular magneto coil. The magneto coil then acts like a battery ignition coil and produces high tension impulses, which are distributed through the distributor rotor and distributor block, and cables to the spark plugs. These high-tension impulses are produced during the engine time that the magneto contact points are open. When the contact points are closed, sparks cannot be generated, although the vibrator continues to sand interrupted current impulses through the magneto contact points without harm to the vibrator or any part of the circuit.
When the ignition switch is in the ON position and the engine starter is engaged, the current from the battery is sent through the coil of a relay which is normally open. The battery current causes the relay points to close, thus completing the circuit to the vibrator coil and causing the vibrator to produce a rapidly interrupted current.
The rapidly interrupted current produced by the vibrator is sent through the primary winding of the magneto coil. By induction, high voltage is created in the secondary winding of the magneto coil, and this high voltage produces high-tension sparks which are delivered to the spark plugs through the magneto distributor-block electrodes during the time that the magneto contact points are open.
The process is repeated each time that the magneto contact points are separated, because the interrupted current once more flows through the primary of the magneto coil. The action continues until the engine is firing because of the regular magneto sparks, and the engine starter is released. It should be understood that the vibrator starts to operate automatically when the engine-ignition switch is turned to the ON position and the starter is engaged. The vibrator stops when the starter is disengaged.
Low Tension Ignition
Reasons for development of low tension ignition
There are several very serious problems encountered in the production and distribution of the high-voltage electricity used to fire the spark plugs of an aircraft engine. High voltage electricity causes corrosion of metals and deterioration of insulating materials. It also has a marked tendency to escape from the routes provided for it by the designer of the engine.
There are four principal causes for the troubles experienced in the use of high-voltage ignition systems:
Is a term to describe the jumping of the high voltage inside a distributor when an airplane ascends to a high altitude. The reason for this is that the air is less dense at high altitudes and hence has less dielectric, or insulating, strength.
Is the ability of a conductor to store electrons. In the high-tension ignition system, the capacitance of the high-tension leads from the magneto to the spark plugs causes the leads to store a portion of the electric charge until the voltage is built sufficiently to cause the spark to jump the gap of a spark plug. When the spark has jumped and established a path across the gap, the energy stored in the leads during the rise of voltage is dissipated in heat at the spark-plug electrodes since this discharge of energy is in the form of a relatively low voltage and high current, it causes burning of the electrodes and shortens the life of the spark plug.
Whenever it exists, increases conductivity. Thus it may provide new and unforeseen routes for the escape of high voltage electricity.
High voltage corona
Is a phrase often used to describe a condition of stress which exists across any insulator (dielectric) exposed to high voltage. When the high voltage is impressed between the conductor of an insulated lead and any metallic mass near the lead, and electrical stress is set up in the insulation. Repeated application of this stress to the insulation will eventually cause failure.
Operation of low tension ignition system
The low tension ignition system consists of a low tension magneto, a carbon brush distributor and a transformer for each spark plug.
During the operation of the low-tension system, surges of electricity are generated in the magneto generator coil. The peak surge voltage is never in excess of 350V and probably is nearer 200 V on most installations. This comparatively low voltage is fed through the distributor to the primary of the spark-plug transformer.
At the instant of opening of the breaker contacts which are connected across the magneto generator coil, a rapid flux change takes place in the generator coil core, causing a rapid rise of voltage in this coil. As has already been explained, it is the capacitor connected across the breaker points which actually stops the flow of current when the breaker opens.
The primary capacitor and magneto generator coil of a low-tension system are connected through the distributor directly across the primary winding of the transformer coil, Therefore, during the time that the voltage across the primary capacitor is rising, as the breaker points open, the natural tendency is for current to start flowing out through the distributor and the primary of the transformer coil.
When this condition has been achieved, we have the situation of a primary capacitor charged to nearly 2000V connected across the primary of the transformer. The result in a very rapid rise of current in the primary, accomplished by a very rapid change in flux linkages (magnetic field) in both coils. The rapid change in the flux linkages in the secondary induces the voltage which fires the spark plug. As soon as the spark gap has been “broken down” (broken through and ionized), current also starts to flow in the secondary circuit.
Duration of the spark in a high-tension system is several times that of a comparable low-tension system. The high resistance of the transformer primary winding, which is characteristic of all low-tension transformer coils, helps to bring the primary current to a stop after the spark has been produced.
It should be clear that the spark voltage is produced by the growth of magnetic field in the transformer core and not by the collapse of the field as is the case in conventional ignition coils. This fact sometimes raises the question about why the subsequential collapse or decay of the field in the transformer does not produce a second spark at the spark plug. The reason for this is that the rate of decay of the magnetic field in the transformer is determined by the rate decay of the primary current. It has already been pointed out that the primary current results from the discharge of the primary capacitor and that this current tapers off at a rather slow rate after the secondary current stops. Since the rate of decay of the magnetic field is the same as that of the primary current, it is too slow to produce enough voltage for a second spark at the plug.
Ventilation and Cooling System
Aircraft engines may be cooled either by air or by liquid, Excessive heat is undesirable in any internal-combustion engine for three principal reasons:
It adversely affects the behaviour of the combustion of the fuel-air charge
It weakens and shortens the life of the engine parts
It impairs lubrication
If the temperature inside the engine cylinder is too great, the fuel mixture will be preheated and combustion will occur before the proper time. Premature combustion causes detonation “knocking,” and other undesirable conditions. It will also aggravate the overheated condition and is likely to result in failure of pistons and valves.
The strength of many of the engine parts depends on their heat treatment. Excessive heat weakness such parts and shortens their life. Also, the parts may become elongated, warped, or expanded to the extent that they sieze or lock together and stop the operation of the engine.
Excessive heat “cracks” the lubricating oil, lowers its viscosity, and destroys its lubricating properties.
Approximately one third of the energy produced by burning fuel in the engine cylinders manifests itself as heat which is not converted to power. If this heat were not dissipated failure of some of the engine components indirect contact with the combustion process would take place, and the engine would fail.
Some of the heat is rejected with the exhaust gases but the remainder must be dissipated so as to maintain the working parts of the engine at a temperature which will ensure that the materials are not adversely affected. However a minimum temperature must be maintained to assist proper lubrication and to provided good fuel evaporation.
There are two main methods of cooling, by liquid or by air, but some internal parts are also cooled by heat transfer through the medium of the lubricating oil.
In liquid cooled engines the cylinders are surrounded by a water jacket, through which liquid (normally a mixture of ethylene glycol and water) is passed to absorb and remove excess heat. The jackets are parts of a closed system, which also includes an engine driven pump and a radiator which projects into the airstream. Some systems are provided with thermostatically controlled radiator shutter, by means of which a suitable coolant temperature is maintained during flight. Liquid cooling has been used mainly on military aircraft engines, but a few examples may still be for on civil aircraft.
With air cooled engines, all those parts of the engine which needs to be cooled (mainly the cylinders) are provided with fins, the purpose of which is to present a larger cooling surface to the air flowing round them. The size of the fins is related directly to the quantity of heat to be dissipated, thus the fins on the cylinder head gave a greater area than those on the cylinder barrel. Baffles and deflecters are fitted round the cylinders to ensure that all surfaces are adequately cooled and the whole engine is cowled to direct airflow past the cylinder to reduce drag. The exit path from the cowling is generally provided with gills or flaps, by means of which the mass air flow may be adjusted to control cylinder temperature. Because air cooling is simple and little maintenance is required, air cooled engines are used in the majority of piston engine aircraft.