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100+ Free Diesel Mechanic Trade Test Practice Questions

South Africa Artisan Trade Test - Diesel Mechanic practice questions are available now; exam metadata is being verified.

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2026 Statistics

Key Facts: Diesel Mechanic Trade Test Exam

100

Practice Questions

OpenExamPrep

2+ days

Trade Test Duration

QCTO/NAMB

NQF 4

Qualification Level

SAQA

QCTO/NAMB

Administering Bodies

DHET

Theory + Practical

Assessment Format

QCTO/NAMB

Competent

Result Standard

QCTO/NAMB

The South African Diesel Mechanic artisan trade test is the external summative assessment under the Quality Council for Trades and Occupations (QCTO) and the National Artisan Moderation Body (NAMB) that certifies a qualified diesel artisan. It is an integrated trade test of written theory and practical tasks conducted over a minimum of two days at a QCTO-accredited trade test centre, where a NAMB-registered assessor records a result of Competent or Not Yet Competent rather than a published question count. Candidates typically enter via a completed apprenticeship or learnership, or an N2 certificate with relevant trade-theory subjects plus work experience. The trade theory spans diesel engine theory, fuel injection systems, drivetrain and transmission, brakes, steering and suspension, hydraulics, electrical and diagnostics, and workshop safety. This free prep includes 100 research-based practice questions with explanations and an AI tutor.

Sample Diesel Mechanic Trade Test Practice Questions

Try these sample questions to test your Diesel Mechanic Trade Test exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.

1What ignites the fuel in a compression-ignition (diesel) engine?
A.A spark from a spark plug
B.The heat of air compressed in the cylinder
C.A glow plug burning during normal running
D.An electrical arc across the injector tip
Explanation: A diesel is a compression-ignition engine: air alone is drawn in and compressed to a high ratio (typically 14:1 to 22:1), raising its temperature to roughly 700-900 degrees Celsius. Atomised fuel injected near top dead centre self-ignites from this heat of compression, so no spark plug is needed.
2In a four-stroke diesel engine, in which order do the strokes occur?
A.Induction, power, compression, exhaust
B.Induction, compression, power, exhaust
C.Compression, induction, exhaust, power
D.Power, exhaust, induction, compression
Explanation: The four-stroke cycle is induction (air drawn in), compression (air compressed and heated), power (fuel injected, burns and forces the piston down), and exhaust (burnt gases expelled). The crankshaft makes two revolutions per complete cycle.
3Why does a diesel engine use a much higher compression ratio than a petrol engine?
A.To reduce the amount of air needed
B.To generate enough heat to self-ignite the fuel
C.To prevent the fuel from atomising
D.To allow the use of a carburettor
Explanation: A diesel relies on the heat produced by compressing air to ignite the injected fuel. High ratios of around 16:1 to 22:1 raise air temperature above the fuel's auto-ignition point. The higher ratio also gives diesels their characteristic high thermal efficiency and torque.
4The point in the cycle when the piston is at the very top of its travel is called:
A.Bottom dead centre (BDC)
B.Top dead centre (TDC)
C.Mid-stroke
D.Valve overlap
Explanation: Top dead centre (TDC) is the highest position the piston reaches in the cylinder, where the swept volume is smallest. Fuel injection timing in a diesel is referenced to a number of degrees of crankshaft rotation before TDC on the compression stroke.
5What is the main function of a glow plug in a diesel engine?
A.To ignite the fuel during every power stroke
B.To pre-heat the combustion chamber air to aid cold starting
C.To measure exhaust gas temperature
D.To recover energy from the exhaust
Explanation: A glow plug is an electrically heated element in the combustion chamber or pre-chamber that warms the air to assist ignition when the engine is cold. Once the engine is warm and running, compression heat alone is sufficient and the glow plug is no longer needed.
6A direct-injection (DI) diesel engine injects fuel:
A.Into a separate pre-combustion chamber
B.Directly into the main combustion chamber above the piston
C.Into the inlet manifold
D.Into the exhaust stream
Explanation: In a direct-injection diesel the injector sprays fuel straight into the main combustion chamber, usually a bowl formed in the piston crown. DI engines are more fuel-efficient than indirect-injection (IDI) designs, which use a separate pre-combustion or swirl chamber.
7What does the cetane number of diesel fuel indicate?
A.Its resistance to knock in a petrol engine
B.Its ignition quality, or readiness to self-ignite
C.Its sulphur content
D.Its viscosity at low temperature
Explanation: Cetane number measures the ignition quality of diesel fuel: a higher cetane rating means a shorter ignition delay and easier, smoother combustion. South African automotive diesel typically has a minimum cetane number of around 45-51.
8Ignition delay in a diesel engine is best described as the time between:
A.Inlet valve opening and closing
B.Start of injection and start of combustion
C.Exhaust valve opening and TDC
D.Two consecutive power strokes
Explanation: Ignition delay is the short interval between the start of fuel injection and the moment the fuel actually begins to burn. An excessively long delay allows too much fuel to accumulate and burn rapidly, causing the characteristic diesel knock.
9Which component seals the combustion gases in the cylinder and transfers heat from the piston to the cylinder wall?
A.The oil control ring only
B.The compression (gas) rings
C.The gudgeon pin
D.The big-end bearing
Explanation: The compression rings seal combustion pressure above the piston and conduct heat from the piston crown into the cylinder wall. A separate oil control ring below them scrapes excess oil from the bore. Worn compression rings cause low compression and blue exhaust smoke.
10What is the primary purpose of a turbocharger on a diesel engine?
A.To cool the engine oil
B.To force more air into the cylinders, increasing power
C.To filter the diesel fuel
D.To reduce the compression ratio
Explanation: A turbocharger uses energy in the exhaust gases to drive a turbine, which spins a compressor that forces a greater mass of air into the cylinders. More air allows more fuel to be burned, raising power and torque without increasing engine size.

About the Diesel Mechanic Trade Test Practice Questions

Verified exam format metadata for South Africa Artisan Trade Test - Diesel Mechanic is pending. The practice questions above remain available while official exam length, timing, passing score, fee, and administrator details are reviewed.