Engine Classification and Components Questions and Answers

Answer: B

Both valves are closed for 40.97% of the cycle.

A four-stroke cycle takes 720 degrees of crank rotation. Find how long at least one valve is open:

\[ \begin{aligned} \text{inlet open} &= 10 + 180 + 40 = 230^{\circ} \\ \text{exhaust open} &= 25 + 180 + 15 = 220^{\circ} \\ \text{overlap at TDC} &= 10 + 15 = 25^{\circ} \end{aligned} \]

\[ \begin{aligned} \text{any valve open} &= 230 + 220 - 25 = 425^{\circ} \\ \text{both closed} &= 720 - 425 = 295^{\circ} \\ \frac{295}{720} &= 40.97\% \end{aligned} \]

Forgetting to subtract the 25-degree overlap gives 270 degrees, or 37.5%, which is not offered. The overlap counts once only, because during it both valves are open together.

Answer: B

A compression ignition engine is a diesel engine.

In a diesel engine only air is drawn in and compressed to a high ratio, about 14:1 to 22:1, which heats it above the self-ignition temperature of the fuel. Diesel injected into this hot air ignites by itself, without a spark, hence 'self ignition' or compression ignition (CI). Farm tractors almost all use CI engines.

A petrol engine is a spark ignition (SI) engine: the air-fuel mixture is ignited by a spark plug. A steam engine is an external combustion engine and has no ignition in the cylinder at all.

Answer: D

The part that is not in the valve operating mechanism is the crank shaft.

The valve train of an overhead-valve engine is: camshaft and cams, tappets (cam followers), push rods, rocker arms, and the valves with their springs. The cam lifts the tappet, the push rod transmits the motion to the rocker arm, and the rocker arm opens the valve.

The crankshaft converts the reciprocating motion of the pistons into rotation. It drives the camshaft through timing gears or a chain, but it is part of the engine's main moving assembly, not of the valve mechanism itself.

Answer: B

The shaft is the cam shaft.

The camshaft carries a cam for each valve. As it turns, each cam lifts its follower at the right moment, opening the inlet or exhaust valve through the push rod and rocker arm, and the valve spring closes it again. In a four-stroke engine the camshaft turns at half the crankshaft speed, since each valve opens once every two revolutions.

The crankshaft drives the camshaft but does not act on the valves directly. A rocker shaft only carries the rocker arms as a pivot; it does not decide the timing.

Answer: B

The keyed answer is Simple ignition system.

A battery ignition system uses a battery, an ignition coil, a contact breaker and a distributor to produce the spark; its usual textbook name is the coil ignition system, which is not among the options. Of the names offered, the key takes 'simple ignition system' as the alternative name.

The others are clearly wrong. The magneto ignition system is a different system that generates its own current and needs no battery. Compression ignition is how diesel engines ignite fuel, by the heat of compressed air, with no spark at all, and there is no 'oil ignition system'.

Answer: A

The voltage is in the range of 6000 v and above.

The air-fuel mixture in the gap between the spark plug electrodes is compressed and a good insulator, so a very high voltage is needed to ionise it and make a spark jump. The ignition coil steps up the 6 or 12 V of the battery to several thousand volts, typically well above 6000 V and up to around 20,000 V.

The other ranges, from 100 V to 5500 V, are too low to break down the compressed mixture in the plug gap, which is why the coil's step-up is needed.

Answer: B

The load is transmitted by the connecting rod.

The connecting rod's small end is attached to the piston through the gudgeon pin and its big end to the crank pin. It carries the force of the gas pressure on the piston to the crank, converting the piston's reciprocating motion into the crankshaft's rotation.

The gudgeon pin only joins the piston to the connecting rod, the piston receives the gas force, and the crank ('Crawk' in the paper) is where the load is delivered, not the part that carries it there.

Answer: C

The thermal efficiency of a two stroke engine is less than that of a four stroke engine.

In a two stroke engine the exhaust and fresh charge overlap during scavenging, so some fresh charge escapes with the exhaust and some exhaust stays in the cylinder. The effective compression stroke is also shorter because the ports occupy part of it.

These losses lower its thermal efficiency, even though it gives more power for its size, so 'equal to', 'greater than' and 'three times more' are all wrong.

Answer: A

Theoretically the inlet valve opens at Top Dead Centre (TDC).

On the theoretical valve timing diagram of a four stroke engine, the suction stroke runs from top dead centre to bottom dead centre, so the inlet valve opens at TDC and closes at BDC. In actual engines it opens a little before TDC to make use of the inertia of the gases.

BDC is where the inlet valve closes theoretically, and 'side centre position' and 'bottom side position' are not standard piston positions.

Answer: D

The compression ratio of a diesel engine is of the order of 15 to 25.

A diesel engine ignites its fuel by the heat of compression alone, so the air must be compressed enough to raise its temperature above the self-ignition temperature of diesel. That needs compression ratios of roughly 15 to 25.

5 to 8 and 10 to 14 are typical of petrol (SI) engines, which would knock at higher ratios, and 26 to 30 would bring very high pressures and heavy engines for little gain.