Seibishi 3kyu › Class 3 diesel engine

Class 3 diesel engine (3級ジーゼル・エンジン) exam topics

3級ジーゼル・エンジン has the fewest candidates of the three types, but it matters for anyone working on trucks, buses or construction machines. The mechanical side is close to the gasoline engine; the differences are in combustion and the fuel system.

Last updated: 2026-10-19

How diesel differs from gasoline

Item	Gasoline engine	Diesel engine
Ignition	Spark plug (火花点火ひばなてんか)	Compression ignition (圧縮着火あっしゅくちゃっか)
Compression ratio	Lower	Much higher
Drawn into the cylinder	Air-fuel mixture	Air only
Power control	Throttle controls the mixture	Amount of fuel injected
Cold start aid	No special device	Glow plugs (グロー・プラグ)

Fuel system

Fuel flows from the tank through the filter and feed pump to the injection pump (噴射ふんしゃポンプ) and then to the nozzles (噴射ふんしゃノズル). The textbook covers in-line pumps (列型れつがた), distributor pumps (分配型ぶんぱいがた) and common rail systems (コモンレール式) with ECU-controlled injectors. Remember to bleed the air (エア抜き) after replacing the fuel filter, because air in the fuel line makes the engine hard to start.

Combustion chambers

Direct injection (直接噴射式ちょくせつふんしゃしき) sprays fuel into a bowl in the piston crown and saves fuel. Pre-chamber designs (副室式ふくしつしき) start combustion in a small chamber; they run more quietly but depend more on glow plugs.

Turbocharging and exhaust

A turbocharger (ターボチャージャ) uses exhaust energy to spin a turbine and push more air into the cylinders. An intercooler (インタークーラ) makes the intake air denser.
The main diesel pollutants are NOx and particulate matter (PM). EGR returns some exhaust gas to cut NOx; a DPF traps particulates; some vehicles use SCR with a urea solution.

Shared with the gasoline engine

Engine mechanics, lubrication, cooling, starter, alternator, battery, measuring tools and the law are studied as for gasoline. If you have already studied gasoline, review Class 3 gasoline engine topics and add the diesel-specific parts.

Practise diesel papers

The app has 20 Class 3 diesel engine papers, papers 1–3 free, with an explanation for every item.

VIP unlocks papers 4 to 20 in all three subjects (51 papers), the per-item tips and the detailed explanations for missed items. VIP is bought inside the iOS app with Apple payment, and Apple shows the price. The App Store version is coming soon. You can try the free exams on the web now.

Sources checked on 2026-10-19: the JASPA registration test page and its 2026 (令和8) guide, the JASPA results for the 2025 first session, the JASPA past papers page and the MLIT press release on the new mechanic qualifications. Rules can change, so check the latest version at these sources.

FAQ

If I passed gasoline, is diesel quicker to learn?

Yes, because engine mechanics, lubrication, cooling, starting and charging are much the same. Focus on the fuel system, combustion chambers and diesel exhaust treatment.

What is the Class 3 diesel pass rate?

In JASPA's 2025 first-session results, 270 people sat the written test and 200 passed, a rate of 74.1%. Rates vary from session to session.

Why does a diesel engine need glow plugs?

In cold weather the compressed air may not get hot enough to ignite the fuel. Glow plugs heat the combustion chamber to make starting easier.

Take a 30-item mock paper and see right away which items you missed and why.

Try it free now

Papers 1–3 of every subject are free, with furigana. Opens in your browser, no sign-up.