Ask the Mechanic: Difference between two and four stroke engines


What is the difference between a two-stroke and a four-stroke engine?


Hello Joyce, to understand the answer to this question, you need to be familiar with how combustion engines work during the intake (piston moving down to allow fuel air mixture into cylinder), compression (piston moving back up to compress the fuel air gases) and combustion (ignition of the compressed gases by a spark) phases.

The difference between a two-stroke and a four-stroke engine is how fast the engine combustion cycle occurs, according to the number of times the piston moves up and down during each cycle. A stroke is when the piston moves from top dead centre to bottom bead centre, or vice versa during the combustion cycle.

A four-stroke engine’s piston completes two strokes per revolution, one compression stroke for one exhaust stroke followed by a return stroke for each. The spark plugs are fired once every other revolution, while power is produced every four strokes of the piston. The four stroke engine does not need pre-mixing of fuel and oil, as it has a separate chamber for the oil.

On the other hand, the two-stroke engine has its entire combustion cycle completed with just one piston stroke. Its compression stroke is followed by an explosion of the compressed fuel. The exhaust is let out during the return stroke allowing a fresh fuel mixture to enter the cylinder.

Spark plugs are fired once every single revolution while power is produced once every two-piston strokes.

Two-stroke engines require pre-mixing of oil and fuel. Four-stroke engines are heavier, bigger, quieter and suitable for bigger vehicles while 2 stroke engines are smaller, louder, more environmentally polluting as they release emissions mixed with burnt fuel and oil while they are suitable for smaller applications such as motorcycles.


Hello Paul, after replacing my oil recently with 5W30 oil, my car, a pajero 1999 V45 with a 6G74 GDI, black smoke is coming from the exhaust. What could be the problem?


Hello Ivan, black smoke is usually caused by an engine running on a rich fuel mixture. This means more fuel is delivered to the combustion chamber than can be burnt. This can be a result of worn out spark plugs, faulty ignition system components such as ignition coils, faulty oxygen sensors or at worst, leaking fuel injectors.

Occasionally, a clogged air filter or faulty throttle unit will affect air fuel ratios to cause rich running. Have these checked.


I bought a used 1998 Renault Clio as my first car. Now, I have realised that the vehicle does not have spare parts, headlights, indicators and bumpers. Even the windscreen is hard to find. Please advise.


Hello Tony, Renault has a dealer in Kampala called Victoria motors. Have you contacted them? Perhaps your 1998 model has parts they do not carry as they sell the brand new ones and yours would require making a special order.

One way to find out is by asking. You also have the option to order parts from abroad. The independent windscreen suppliers can also help you find a windscreen for your Clio.


Hello Paul, I have a Chrysler grand voyager 1998 model with a V8 engine. However, the engine is faulty and I am wondering if I can swap it for a V8 diesel engine. I would like to use it for heavy workload.


Hello Steve, the 1998 Chrysler Grand Voyager was built with Mitsubishi engines. This Voyager offered one diesel 2.5 line 4 turbo charged engine. The petrol engines offered were the line 4 2.0 and 2.4 Litre. The punchier V6 engines were 3.0, 3.3 and 3.8 Litres. There is no V8 diesel variant.

The Voyager is a minivan, not a truck suitable for the heavy work you have in mind. It may not be worth the trouble of undertaking the costly conversion to a potentially troublesome used old diesel engine with hard to find parts.

It will be easier to find parts for the petrol engine from US online parts shops.

Send sms: mycar (space) your comments and questions to 6933 or email them to: [email protected]


You're all set to enjoy unlimited Prime content.