Engine Performance Flashcards
Define: Force
Any interaction that, when unopposed will change the motion of an object.
- Force = mass x acceleration (F = ma)
- Newton’s 2nd Law
- Measured in newtons (N)
1N = the force required to accelerate 1kg at 1m/sec/sec (1kg x 1m/sec^2)
Define: Work
Whenever a force moves an object through a distance.
- Work = Force (N) x Distance (M) = (W=FD)
- Measured in Nm (newton metres
- !Nm = 1 Joule (J)
Define: Power
The rate of doing work, how much work is being carried out over a given time.
- Power = Work done/Time taken
Power = (Force x Distance)/Time - Measured in Watts (W)
1W = 1J in 1 second - Engine power is normally given in Kilowatts
Define: Energy
The capacity (or ability) to do work.
- Measured in Joules (J)
- In a piston engine, heat energy in fuel will be converted into mechanical energy.
Types of energy:
- Kinetic - the energy a moving object has.
- Elastic - the energy that a stretched object has
- Chemical - what fuel has before it is burnt
- Thermal - the energy that an object has due to temperature
- Gravitational - the energy stored in an object due to its height above the earth
Define: Torque
The force which produces a turning or twisting motion in an object.
- Measured in Newton metres (Nm)
- Torque = Force x Perpendicular distance
- Torque = Force x Distance = F x D
What is Engine Torque?
Rotational force the engine can produce on the crankshaft. Turns the propeller.
What is propeller torque?
The force that opposes and resists engine torque caused by drag which resists it turning. At a constant RPM, engine torque in balance by being equal and opposite propeller torque
What is horsepower?
Imperial unit used to measure engine power. Kilowatts is more used today, but light aircraft still use horsepower.
What is brake power?
- AKA “Brake Horsepower (BHP)”
- Actual power output at the crankshaft.
- What the engine ‘gives’ to the propeller and in an aircraft.
What is indicated power?
- AKA “indicated horsepower”
- The total power developed within the cylinders by the combustion process.
What is friction power?
Power lost due to friction, and from driving pumps, mags, alternator etc.
- Friction power loss has a steady increase with increased RPM
Brake Power = Indicated Power - Friction Power
What is rated power?
The brake power (BHP) that can be continuously maintained by an engine in:
- Good condition - engine not worn out
- Under standard conditions (ISA)
- At the stated RPM (stated manifold pressure if fitted with CSU)
AKA “Maximum Except for Takeoff (METO_ or “Maximum Continuous Power (MCP)
What is rated/critical altitude?
The altitude at which the engine will develop its rated power at full throttle.
- This occurs only at MSL
What is full throttle height?
The altitude up to which a given manifold air pressure (MAP) set can be maintain at a given RPM.
- Generally used for turbocharged or supercharged aircraft.
What is thermal efficiency?
- AKA “Brake thermal efficiency”
- Overall efficiency of the engine
- Thermal efficiency of a piston engine = 20 to 25%
(Brake) Thermal efficiency = Brake power/fuel consumption
- Can be expressed as how much fuel is being used at any one point in time (fuel flow), to how much power is being produced at that same time (brake power) = “Brake Specific Fuel Consumption” (BSFC)
What is mechanical efficiency?
Part of the total thermal efficiency of the engine. The ratio between indicated power and brake power.
Mechanical efficiency = brake power/indicated power
Brake power = actual power outputted from the engine
Indicated power = total power outputted within the cylinders by the combustion process.
- At the most efficient RPM, it can be 90%
What is volumetric efficiency?
How efficient the engine is at ‘breathing’ or taking in the induction of the fuel-air mixture (charge).
- For it to be 100% efficient, the volume of charge drawn into the cylinder must be equal to the volume displaced by the piston.
The highest volumetric efficiency is when the throttle valve is fully open.
What are factors reducing volumetric efficiency?
- Drag from the walls of the pipes/manifolds
- Inertia of the charge, and rate of inflow (increasing RPM)
How to improve volumetric efficiency?
- Better pipes/manifolds design - avoid friction losses
- Corrects valve timing - valve lead, lag and overlap
- Having cool inlet air - warm air is less dense
Volumetric efficiency formula
Volumetric efficiency = volume of charge/piston displacement
Thermal efficiency mnemonic:
To Fly Bravely
Thermal efficiency = fuel flow/brake power
Mechanical efficiency mnemonic
Means Being Into
Mechanical energy = brake power/indicated power
Volumetric efficiency mnemonic
Vicious Vibrating Planes
Volumetric efficiency = volume of charge/piston displacement
How can an engines efficiency be maximised?
Getting the most brake power for the least amount of fuel.
- High density air - low temp, high pressure, low humidity
- Aircraft at full throttle height
- Properly leaned mixture
- Low RPM and high manifold pressure - least amount of friction
- Carburettor heat off/Cool induction manifold temperature - high air density
If ambient air pressure decreases with an increase in altitude, what will the manifold pressure do?
Also decrease
What does power available refer to?
Power available from the propeller.
- Power availed will increase as the TAS of the aircraft increases
What is the difference between the brake power and power available from the propeller?
Propeller inefficiency (or propeller losses) which is caused by induced drag, shockwaves, and skin friction drag.
What is peak propeller efficiency achieved at for a fixed pitch propeller?
One TAS/RPM
What peak propeller efficiency is achieved in an aircraft with a CSU?
Over a range of speed.
