AGK ENGINES - CHAPTER 2 - 4 STROKE CYCLE

What is the main purpose of advancing ignition timing before TDC?

It allows combustion to occur while the piston is near TDC, so maximum pressure is achieved just as the piston starts its power stroke, increasing engine efficiency.

Why is ignition retarded during engine starting?

Ignition is retarded to around TDC during start to prevent backfiring or reverse rotation at very low RPM.

What can happen if ignition timing is not retarded at low RPM?

If not retarded, peak pressure may occur before the piston reaches TDC, causing inefficiency, potential engine damage, and even reversed crankshaft rotation.

When is ignition said to be advanced?

Ignition is advanced when the spark occurs before the piston reaches Top Dead Centre (TDC) on the compression stroke.

Why is ignition timing advanced as RPM increases?

At higher RPM, the piston moves faster, so ignition must be advanced to allow combustion to reach peak pressure as the piston starts its power stroke.

What is ignition timing in a piston engine?

Ignition timing refers to the precise moment at which the spark occurs to ignite the fuel-air mixture, usually slightly before TDC to maximise efficiency.

Can valve timing be changed with engine RPM in aero engines?

No, in conventional aero engines, valve timing is fixed by the cam profile and does not change with engine RPM.

What is the advantage of a longer valve lead and lag?

A longer valve lead and lag reduces the speed at which the valves must operate, decreasing the stresses on the engine's valve gear and allowing more mixture to enter the cylinder.

What is scavenging in piston engines?

Scavenging is the process during valve overlap when the action of outgoing exhaust gases helps draw in fresh mixture.

What is valve overlap?

Valve overlap is the period when both the inlet and exhaust valves are open at the end of the exhaust stroke and the start of the induction stroke, assisting in drawing fresh mixture into the cylinder.

What is exhaust valve lag?

Exhaust valve lag is when the exhaust valve remains open slightly after Top Dead Centre (TDC), allowing remaining exhaust gases to exit under their own momentum.

What is exhaust valve lead?

Exhaust valve lead is when the exhaust valve opens just before the end of the power stroke to allow more time to clear waste gas from the cylinder.

When are both the inlet and exhaust valves closed in a piston engine cycle?

Both valves are closed during the compression stroke, compressing the mixture for ignition.

How is valve lead and lag measured?

Valve lead and lag are measured in degrees of crankshaft rotation.

What is the inlet valve lead in a piston engine?

Valve lead is the process of opening the inlet valve before Top Dead Centre (TDC), allowing fresh mixture to enter the cylinder before the start of the induction stroke.

How are the valve and ignition timings adjusted to optimise the four-stroke cycle?

By altering when the inlet and exhaust valves open/close (valve timing) and when ignition occurs (ignition timing).

What is valve lag in a piston engine?

Valve lag occurs when the inlet valve remains open beyond Bottom Dead Centre (BDC) and slightly into the compression stroke, allowing the mixture to continue entering the cylinder due to its momentum.

Why does the four-stroke cycle need to be optimised for ineffective crank angle and combustion time?

Because air has mass, inertia, and momentum, and it takes time to completely burn the mixture and for combustion gas to reach maximum pressure.

What is the ineffective crank angle in a piston engine?

It is the period near Top Dead Center (TDC) and Bottom Dead Center (BDC) where there is little linear movement of the piston despite the crankshaft rotating.

What does RPM stand for and what does it measure?

RPM stands for Revolutions Per Minute and measures the number of times the crankshaft rotates in one minute.

What causes air to flow into and out of a four-stroke engine cylinder?

Air flows due to pressure changes produced by the changing volume of the cylinder.

What is a typical cruise RPM setting for a petrol engine?

A typical cruise setting is 2300 RPM, meaning the crankshaft rotates 2300 times every minute.

How many revolutions of the crankshaft are required for a four-stroke cycle?

Two revolutions of the crankshaft are required for a four-stroke cycle.

What happens to pressure and temperature during the exhaust stroke as the piston rises?

Both pressure and temperature increase as the volume decreases.

What is expelled from the cylinder during the exhaust stroke?

The products of combustion (burnt gases) are expelled through the exhaust port to the atmosphere.

What happens at the beginning of the exhaust stroke?

The exhaust valve opens as the piston begins to rise from BDC.

What changes occur to cylinder volume, pressure, and temperature during the power stroke?

Cylinder volume increases, while pressure and temperature decrease.

What happens to the piston during the power stroke?

The rapid rise in pressure forces the piston down the cylinder.

In which stroke is power actually produced in a four-stroke cycle?

Power is produced during the power stroke.

Why does combustion produce a large increase in pressure and temperature?

Combustion occurs in a confined space at TDC, causing a very large increase in pressure and temperature.

What causes combustion in a piston engine?

Combustion is caused by an electrically generated spark from one or more spark plugs at the top of the cylinder.

When does the ignition of fuel occur in the four-stroke cycle?

Fuel is ignited at or near Top Dead Centre (TDC) at the end of the compression stroke.

What happens to the pressure and temperature during the compression stroke in a piston engine?

Both pressure and temperature increase as the volume decreases.

What is an engine called that draws in fuel and air without forced induction?

Normally aspirated engine.

What happens to volume, pressure, and temperature during the induction stroke?

Volume increases, pressure reduces, and temperature reduces.

When does ignition occur in the four-stroke cycle?

Between the compression and power strokes.

How much must the crankshaft rotate to move the piston through one stroke?

180 degrees.

What is the bottommost limit of piston travel called?

Bottom Dead Centre (BDC).

What is the topmost limit of piston travel called?

Top Dead Centre (TDC).

What does “stroke†refer to in an engine?

The distance the piston travels between its lowest and highest points in the cylinder.

What is a simple way to remember the four-stroke cycle?

Suck, squeeze, bang, and blow.

What happens during the power stroke?

The piston is driven down the cylinder to produce mechanical power.

What happens during the compression stroke?

The mixture is compressed to maximise the pressure produced by combustion.

What happens during the induction stroke?

The mixture is drawn into the cylinder.

What happens during the exhaust stroke?

Waste gases are expelled from the cylinder before the cycle repeats.

What are the four operations involved in the four-stroke cycle?

Induction, Compression, Power, Exhaust.

What is the technically correct name for petrol engines?

Spark ignition engine.

What causes the piston to move back up the cylinder after combustion?

The angular momentum of the crankshaft and rotating parts pushes the piston back up.

How is the crankshaft connected to the propeller?

The crankshaft can be connected directly or indirectly (through a gearbox) to the propeller.

How is thrust produced in piston engines?

The propeller accelerates air rearwards, creating an equal and opposite reaction known as thrust.

What is the function of the exhaust valve and exhaust port?

The exhaust valve opens to allow combustion products to exit through the exhaust port.

What is the function of the inlet port and inlet valve?

The inlet port introduces the mixture into the cylinder, and the inlet valve closes the port to seal the cylinder.

Why is a piston engine called a reciprocating engine?

Because the piston moves up and down (reciprocates) in the cylinder.

What motion does the crankshaft convert?

The crankshaft converts the piston's linear (up and down) motion into rotary motion.

How is the charge ignited in petrol engines?

By an electric spark plug.

What is the charge in a piston engine?

The total amount of mixture (air and fuel) drawn into a cylinder.

What is the mixture in a piston engine?

Atmospheric air mixed with fuel.

What are the products of combustion called?

Exhaust gas or products of combustion.

What is the working fluid in internal combustion engines?

Air is the working fluid and the term “air†covers all the various forms of gas in the engine.

How is combustion initiated in diesel engines?

Combustion is initiated by the heat generated from compressing the mixture.

How is combustion initiated in petrol engines?

Combustion is initiated by an electrically generated spark.

What drives the propeller in a piston engine?

Mechanical energy from the piston accelerates air rearwards via the propeller to produce thrust.

How is energy released in a piston engine?

Fuel is burned with air in a confined cylinder to release energy as heat, increasing gas pressure.

What are the four types of energy conversions in a piston engine?

Chemical energy, heat energy, pressure energy, and mechanical energy.