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ASVAB · Engine Basics and the Four-Stroke Cycle

What is the difference between how a gasoline engine and a diesel engine ignite their fuel?

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Correct answer

A gasoline engine uses a spark plug to ignite the mixture; a diesel engine ignites fuel by the heat of high compression, with no spark plug

  1. A They ignite identically
  2. B Diesel uses spark plugs, gasoline uses compression
  3. C A gasoline engine uses a spark plug to ignite the mixture; a diesel engine ignites fuel by the heat of high compression, with no spark plug
  4. D Neither uses combustion

Why this is the answer

A gasoline engine uses a spark plug to ignite the compressed air-fuel mixture at the right moment. A diesel engine has no spark plugs; instead it compresses air to a very high pressure, which makes it hot enough that injected diesel fuel ignites on contact (compression ignition). Diesel engines run at higher compression ratios and are generally more fuel-efficient and produce more low-end torque, while gasoline engines rev higher and run more smoothly at high speed. Knowing this key difference — spark ignition versus compression ignition — is a frequently tested engine-basics concept.
Source: ASVAB Auto Information — Gasoline vs Diesel Ignition

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About this topic: Engine Basics and the Four-Stroke Cycle

Engine basics cover the four-stroke cycle (intake, compression, power, exhaust), the parts that make it happen — pistons, valves, crankshaft, camshaft — and the terms used to describe engines, such as displacement and compression ratio. This is one of 21 Auto Information questions on engine basics and the four-stroke cycle, each with a full explanation on a single study page.

New to this exam? Our ASVAB exam guide explains the format, scoring, and how to prepare.

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