Propulsion -- Turbo, Super, and Exhaust Systems

Manifold Absolute Pressure

-(MAP) is the pressure in the intake manifold of an engine -The amount of the charge entering the cylinders is measured by MAP and temperature -In a normally aspirated engine, MAP is less than atmospheric pressure -This is due to friction losses within the induction system

A complete induction system includes how many sections?

-3 sections 1.Air scoop and ducting leading to the carburetor 2. The carburetor, or air control section, of an injection system 3. The intake manifold and pipes -The above are the pathways for all the air which must be supplied to the engine -We will talk about the carburetor and injection system in following chapters

Turbocharger

-A turbocharger is an externally driven device designed to be driven by a turbine wheel -Receives its power from the engine exhaust -Ram air is applied to turbocharger inlet -Turbocharger outlet to the carburetor (or fuel injector) -Sometimes it is necessary to cool the compressed air that leaves the turbocharger before it enters the carburetor or detonation may occur -Turbine - a rotating device turned by an outside force such as air or water flowing past it to transform energy into power or to drive a compressor. -Impeller - The main rotor of a radial compressor which increases the velocity of the air being pumped.

Manuel Waste Gate Control

-Control in the cockpit -Usually with after market or retrofit installations -Takeoff is accomplished with no boost -When the pilot observes the MAP dropping with full throttle then the waste gate is adjusted -When the waste gate is completely closed and the MAP still drops then the engine is at its critical altitude -For descent, the process is reversed -This type of system creates a high pilot workload

Carb Heat System

-Engine air is allowed to flow between the exhaust pipe and the shroud -The air is heated before being sent to the carburetor inlet -This is used to prevent icing -Use of carburetor heat results in a loss of power because warmer air is less dense than cool air -Prolonged use may also cause detonation

Air Cooling

-Engines that are air cooled use thin metal fins that project from the outer walls and surfaces of the cylinders -As air flows over the fins, it absorbs excess heat via conduction -Some cylinders will have deflector baffles which are used to direct the flow of air for maximum cooling effect -Proper installation of the baffles is essential or hot spots may develop (causing engine failure) -Engines may also be cooled through the use of cowl flaps -These are openings in the engine cowling that may be either automatically or manually activated -The majority of cooling, however, is obtained when the exhaust gases are vented to the atmosphere -40-50% of the heat is disposed of this way -Proper exhaust valve timing dictates how successful this is

Intake Manifold and Piping Section

-In other engines, the charge is heated via proximity to the exhaust manifold -It is essential that there be a gastight seal between the carburetor and the cylinders -Manifold absolute pressure (MAP) is below atmospheric in unsupercharged engines -An air leak will thin the charge -This may cause engine overheating, detonation, preignition, or stoppage

Liquid Cooling

-Liquid cooling involves liquid passages around the cylinders and hot spots of the engine -Water was initially used for a coolant but now it is usually a combination of water and ethylene glycol -Although rarely used in the US today, the development of new liquid-cooled engines is ongoing

Automatic Control

-Maintains deck pressure -Deck Pressure is the pressure between the discharge of the compressor and the engine throttle valve -Maintains it at 28 to 30 in hg in a normalized engine or higher for a ground boosted engine -Ground boosted engines require in excess of sea level pressure to produce its max rated HP

Normalizers

-Some engines have air compressors that raise the pressure, but not above 30 in Hg -They will be used to compensate for decreased air density at altitude, not "supercharging"

Waste Gate

-The exhaust gases are usually diverted to the turbochargers via a waste gate -The degree to which the waste gate is opened determines the degree of turbo-charging -Turbochargers are designed to deliver excess power up to their critical altitude -After the critical altitude, the charger will no longer deliver the full power for which it was designed

The capacity of the supercharger depends on...?

-The size of the impeller and -The speed at which it is rotated -Turbochargers depend upon the amount of exhaust gases, also

Reciprocating Cooling Systems

-There are three main reasons excessive heat is harmful in a reciprocating engine 1. Adversely affects the combustion of the fuel-air mixture - Mixture preheats and combustion occurs prematurely 2. Weakens and shortens the life of the engine - Parts may also warp and the engine may seize 3. Impairs lubrication - The viscosity of the oil is reduced

Induction System Icing

-This hazard may cut off the flow of the fuel-air charge or vary the fuel-air ratio -It may occur when the aircraft is in clouds, fog, rain, snow or even clear air -Generally it is classified into three types 1. Impact Ice 2. Fuel Evaporation Ice 3. Throttle Ice -We will cover in greater detail when we discuss carburetors and fuel-injection systems -Ice can form anywhere from the air scoop to the intake manifold -Icing can be detected by a reduction in engine power when the throttle remains fixed -Fixed-pitch props will see a decrease in engine rpm -Constant-speed props will see a decrease in manifold pressure and a decrease of engine power, with constant rpm -Aircraft that will normally operate in icing conditions should be equipped with a carburetor air temperature (CAT) gage -Reads the temp of air as it enters the carburetor -If below 32°F and there is loss of power, assume icing exists and apply carburetor heat

Engine Exhaust Systems

-This is one of the most critical systems in proper engine operation -The products of combustion from the engine are both toxic and very hot -Early exhaust systems consisted of simple vents from the exhaust ports -Pilots would determine how hot their engine was running by looking at the color of their exhaust flame -The next generation of exhaust systems introduced the exhaust manifold -This device directs gases out and downward, away from the cockpit or cabin -The third generation of design involved the addition of muffs and other heat-exchanger equipment -Modern aircraft use a combination of 2nd and 3rd generation systems -Modern aircraft use some of the hot exhaust gases for -Cabin heating -Carburetor anti-ice -Windshield defrost -Turbochargers

Turbochargers for Lightweight Aircraft

-This particular engine turbocharger makes it possible to maintain a sea level pressure all the way to 20k ft of altitude -Limitations listed for turbocharger use will be -Maximum and minimum engine RPM -Maximum MAP -Maximum Cylinder Head Temperature (requires an additional gage) -Range for carburetor inlet air temperature

Turbine Impeller

-a rotating device turned by an outside force such as air or water flowing past it to transform energy into power or to drive a compressor. -a rotating device used to force a fluid in a desired direction under pressure, can be the main rotor of a radial compressor which increases the velocity of the air being pumped.

The carburetor air box houses...

-a swing check valve -This valve is open during normal operation (without the turbocharger) allowing ram air, but once turbo boosts air the valve closes. -The valve movement is automatic -Lubrication for the turbocharger is accomplished at a lower pressure than standard for the engine -There is a warning light if the turbocharger oil drops below a pre-specified level.

What is the advantage of liquid vs air cooled?

-air does not have a high heat density -In other words, volume for volume, the liquid can remove more heat from the engine -Also, it is easier to get the liquid to the hot spot -Oftentimes this area on the engine is hidden away from the air stream

Most superchargers use...

-an impeller rotating at high speed to compress either the air before it is mixed with the fuel in the carburetor or the fuel-air mixture as it leaves the carburetor -Internal-type supercharger: supercharger is between the carburetor and cylinder intake ports (compress the charge)

Air Scoop and Ducting Section

-for a nonsupercharged engine this system has four principle parts 1. Air Scoop 2. Air Filter 3. Alternate Air Valve 4. Carburetor Heat Air Valve, or Heater Muff

In a supercharged engine, MAP may be...?

-higher than atmospheric pressure -It may also be less than atmospheric, depending on setting -In engines associated with constant-speed propellers, if MAP becomes too high detonation and overheating will occur

Supercharger Turbocharger

-is an air compressor used to force more air into the combustion chamber of an internal combustion engine than can be achieved with standard atmospheric pressure -are superchargers that are turbine-driven -The more correct name is turbo-supercharger if they boost MAP over 30"hg

As air is compressed...

-its temperature rises -The rising temperature reduces supercharger efficiency because heated air also expands -The expansion increases the amount of power required to compress it -Overall engine efficiency also decreases with a warmer charge

Density Controller Differential Pressure Controller

-limits maximum MAP to prevent over-boost by limiting the deck pressure while the aircraft is below the engines critical altitude -regulates the position of the waste gate valve to maintain a pre-set differential across the throttle; usually 2 to 3in

The internal supercharger may be used on what kinds of engines?

-lower altitude engines or where the carburetor does not require pressurized air -External-type supercharger: supercharger is before the carburetor (compress the air) -The external superchargers are normally turbine driven -If the air compresses only to 30 in Hg, it is called a turbocharger rather than turbosupercharger

Superchargers allow an engine to develop what?

-maximum power when operating at high altitudes -They are used to overcome the decrease in air density as an aircraft gains altitude -They may also be used to boost power on takeoff -Superchargers usually consist of a centrifugal compressor which is connected to the air intake of the engine -The compressor may be driven -By gearing from the crankshaft -By exhaust gases -An aircraft is supercharged when the manifold absolute pressure (MAP) is over 30 in Hg -Some engines may be boosted to 34-48 in Hg

If the temperature of the charge gets too high, what can happen?

-pre-ignition and detonation may take place -The above results in power loss or complete mechanical failure -The octane of the fuel used in supercharged engines must accommodate not only normal compression but also the additional compression of supercharging

If other factors remain the same, the gain in power output from the engine is...?

-proportional to the increase in pressure -An aircraft at sea level which increases pressure from 14.7psi to 20psi will produce 40% more power -The pressure can not be increased indefinitely or permanent engine damage may result

Superchargers can be classified as...

-single-stage, two-stage, or multi-stage -Stage equates to how many times the pressure is increased -Superchargers may also be multi-speed -Since the impeller will rotate much faster than the crankshaft, sometimes it has adjustable speeds -It is possible to have a two-speed, single stage supercharger

Variable Pressure Controller

-takes the place of a density and differential pressure controllers and reacts to the pressure of the air between the compressor and the throttle valve (deck pressure) to control both max MAP and regulate the differential. -Often will have a "Cam follower" to help present the controller for the throttle setting. -Some installations have an Over-boost Control Valve that is simply a spring operated valve in the intake valve that opens when the pressure gets too high

If the power to run the supercharger is coming from the crankshaft...

-the net gain in horsepower is reduced -The net gain from the supercharger is also offset by the added weight of the system -Special cooling systems may also be required with superchargers, which also add weight and size (spacing)

Intake Manifolds

-these are what send the pressurized fuel-air mixture into the cylinders -The typical opposed-engine has an induction system with an individual pipe leading to each cylinder -Sometimes the piping extends through the oil sump, which serves to -Cool the oil slightly -Increase the temperature of the charge for better vaporization of the fuel

Alternate Air Valve

-this is designed to allow air to get to the engine if the filter or other parts of the induction become clogged -Can be manually or automatically controlled -Alternate air source is either unfiltered warm from the engine or outside (cold) unfiltered air -Sometimes this valve will be used in heavy rain to allow normal operation of the engine -The alternate air valve may also be used to allow warm air to enter the induction system -The above counters carburetor icing

Air Filter

-this is installed at or near the air scoop in order to remove dirt, abrasive particles, and sand before they are carried into the engine -Air filters will reduce air pressure somewhat, thus reducing power output -There are three basic types of air filters

Carburetor Heat Air Valve and Heater Muff

-this is operated from the cockpit via the carburetor heat switch -When turned off (normal operation), the passage to the heater muff is closed and the main air duct is open -When turned on, a valve closes the main air duct and opens a duct to the heater muff -The heater muff is a shroud placed around a section of the exhaust pipe

Air Scoop

-this is simply the opening facing the airstream -The scoop receives ram air (usually augmented by the propeller slipstream) -The effect of the added air velocity is to "supercharge" (compress) the air a small amount -This compression adds to the total weight of air received by the engine -The power increase may be as much as 5% depending upon the scoop design

What is the main purpose of supercharging?

-to increase MAP above atmospheric -Increasing MAP increases the power output in two ways 1. It increases the weight of the charge delivered to the cylinders of the engine 2. It increases the compression pressure - This increases BMEP and thus power

Most engines with high compression ratios can not what?

-use superchargers until they are flying at an altitude of 5000' or higher -Otherwise the high pressures and temperatures in the combustion chambers will cause detonation and pre-ignition -Higher octane fuels allow for greater supercharging

Overboost Overshoot Bootstrapping Turbine Inlet Temperature (TIT)

-when MAP exceeds the limits at which the engine was tested and certified by the FAA -occurs when the automatic controls do not respond quickly enough to stop the inertia of the turbocharge speed increase with a rapid throttle increase -an unregulated power change that results in the continual drift of manifold pressure -Measurement of the EGT at the Turbocharger turbine Inlet

An engine with a supercharger must be able to...

-withstand higher stresses developed with the increased power -The engine will also have to develop more power to actually drive the compressor of the supercharger itself

Advantages to air cooling

1. Weight of the engine is less than if liquid is used 2. The engine is less affected by cold-weather operations 3. For military operations, this engine is less susceptible to damage from gunfire