Combustion in CI Engines Questions and Answers

Answer: A

The true statement for an SI engine in comparison with a CI engine is that its operating speed is high. SI engines have light parts and a lower compression ratio and so run at higher speeds. In an SI engine fuel is lost in scavenging in the two stroke type, starting is easy and the idle noise is less, so the other statements are the reverse of the truth.

Answer: D

The parameter that is low for a CI engine compared with an SI engine is the exhaust gas temperature. A CI engine has a high expansion ratio and works with lean mixture, so the gases leave at a lower temperature. A CI engine has more noise, needs more cranking effort because of the high compression ratio, and is heavier per unit of power, so the other three parameters are higher.

The exhaust is cooler because the gases expand through a higher ratio, so more of their heat has been turned into work before they leave.

Answer: D

Compared with the spark ignition engine, the diesel engine has lower power and lower exhaust gas temperature.

A diesel engine must run with considerable excess air to burn its fuel cleanly, so for the same swept volume and speed it burns less fuel per cycle than a petrol engine working near the chemically correct mixture. Its power output per unit displacement is therefore lower, which is why diesel engines are built heavier and larger for the same power.

Its exhaust is cooler because of the much higher compression and expansion ratio: the gases expand further and do more work before the exhaust valve opens, and the excess air dilutes them. The pairs with 'higher' get one or both of these comparisons the wrong way round.

Answer: B

The match is P-4, Q-1, R-2, S-3.

• Cetane number measures the ignition quality of diesel fuel.
• A pyrheliometer, pointed at the sun through a narrow tube, measures direct beam radiation.
• Octane number measures the anti-knock quality of petrol.
• A pyranometer, open to the whole sky, measures total (global) solar radiation.

The other options swap the cetane and octane numbers or the two radiometers.

Answer: A

The temperature is the pour point.

The pour point is the lowest temperature at which a fuel or oil still flows (pours) under standard test conditions; below it, wax crystals and rising viscosity stop the fuel flowing. It matters for diesel used in cold weather, because a fuel below its pour point cannot reach the injection pump.

The cloud point is the higher temperature at which wax first appears as a haze, while the fuel still flows. The flash point is the lowest temperature at which the fuel gives off enough vapour to ignite momentarily in air, a safety property for storage. The boiling point is where the liquid boils, which is unrelated to cold flow.

Answer: C

The most important property is the pour point.

The pour point is the lowest temperature at which a fuel still flows. In very cold climates, diesel's waxy components crystallise as the temperature drops, the fuel thickens, and filters and lines can block so that the engine is starved of fuel. A fuel for such regions must have a pour point (and cloud point) below the lowest expected temperature, which is why winter grades of diesel are blended.

Heating value governs energy content and fuel consumption, flash point is a safety property for storage and handling, and boiling point (volatility) affects vaporisation, but none of these stops the fuel flowing at low temperature.