Piston Engine Fundamentals Flashcards
What is Top Dead Centre (TDC)?
When the piston is at the top of its stroke
What is Bottom Dead Centre (BDC)?
When the piston is at the bottom of its stroke
Define what is “stroke” in piston engine theory.
The linear distance the piston travels inside the cylinder from TDC to BDC
Define “clearance volume”.
The volume of the area above the above the piston when at TDC
What is swept volume?
The volume of a cylinder between the TDC and BDC
What is swept volume also known as?
Displacement volume
Define piston displacement.
The total volume swept by all of the pistons as they move through one revolution of the crankshaft
What is compression ratio?
The ratio of cylinder volume at bottom dead centre and top dead centre
What is the formula for compression ratio?
(check answer with pg 15 of PDF)
What factors affect engine efficiency?
Thermal, volumetric and mechanical efficiency
What is thermal efficiency?
It is the ratio of useful work done to the heat energy of the fuel used
What is the formula for thermal efficiency?
(check answer with pg 16 of PDF)
What is volumetric efficiency?
It is the ratio of the amount of air the engine takes into the cylinder to the total volume of the cylinder
How do you calculate volumetric efficiency?
(check answer with pg 17 of PDF)
What factors affect volumetric efficiency? (List at least 3)
Part throttle operation, long intake pipes with small diameter, sharp bends in induction system, high carby air temps, high cylinder head temps, incomplete scavenging, improper valve timing
What is mechanical efficiency?
It is the ratio of brake horsepower to indicated horsepower
What is the formula for mechanical efficiency?
(check answer with pg 18 of PDF)
If an engine develops 160 BHP and 180 IHP, what is the ratio? And what is the mechanical efficiency expressed as a percentage? (use formula to calculate)
The ratio is 160:180
The mechanical efficiency expressed as a percentage is 89%.
What factor has the greatest affect on mechanical efficiency?
Friction
What is 1 BTU?
The quantity of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit.
How many joules is 1 BTU equal to?
1055 joules
What is Brake Horsepower (BHP)?
It is the actual amount of power delivered to the propeller shaft
What does effective combustion require?
A metered combustible gas, compression of the gas, a source of ignition and an avenue of escape for the burnt gas
True or False: A two stroke engine fires on every second revolution of the crankshaft.
False. It fires on every rotation of the crankshaft.
What are the strokes of a two-stroke engine called?
Compression and Power
What are the strokes of an Otto Cycle engine (4-Stroke)?
Intake, compression, ignition and exhaust
What is the number of crankshaft degrees that the intake valve opens BEFORE TDC called?
Inlet valve lead
What could happen if the intake valve didn’t open at the correct time?
The air-fuel ratio will be affected causing the engine to run rough or not at all
When does the intake valve open?
Approx. 15 degrees before TDC (inlet valve lead)
When does the intake valve close?
Approx. 60 degrees past BDC (inlet valve lag)
What are the advantages of inlet valve lag?
It helps improve volumetric efficiency by allowing a greater charge of air/fuel mixture into the cylinder.
When does the exhaust valve open?
Approx. 60 degrees before BDC (exhaust valve lead)
When does the exhaust valve close?
Approx. 10 degrees past TDC (exhaust valve lag)
What are the advantages of exhaust valve lead?
Improve volumetric efficiency and help remove heat from the cylinder
What are the advantages of exhaust valve lag?
Helps take advantage of exhaust gas inertia
Why is does ignition occur before TDC? (ignition advance)
So that pressure can be applied to the cylinder at the correct angle.
Approximately when is ignition initiated?
20-30 degrees before TDC
What is valve overlap and for how long does it occur?
The point in time in which both valves are open. This happens for approximately 25 degrees.
What is another term for a diesel engine?
Compression-ignition engine
What is the difference between an Otto cycle engine and a Diesel engine?
Compressed hot air ignites the fuel rather than spark plugs
What compression ratio is typical for a diesel engine?
Between 14:1 and 24:1
Is premature detonation an issue for diesel engines? Why/Why not?
It is not an issue for diesel engines as the fuel is not introduced into the cylinder until shortly before TDC
What are some advantages of diesel engines?
More fuel efficient, longer life span, absence of coils and spark plug leads eliminates a source of radio frequency emissions
What are some disadvantages of diesel engines?
Heavier, more prone to vibration and higher initial cost.
What configuration do opposed engines have?
Odd numbered cylinders on the right and even numbered cylinders on the left?
What configuration do single row radials have?
They are numbered consecutively starting from the top cylinder and progressing clockwise (as viewed from the pilot’s seat)
What configuration do double-row radial engines have?
Numbered consecutively starting from the top cylinder progressing clockwise. Odd numbered cylinders are in the rear and even numbers are in the front.
What is the firing order for single row radials?
Odd numbers first then even numbers
