Scooter HQ clear the confusion. Two stroke or Four Stroke There is much debate about two stroke and four stroke engines which is causing a great deal of confusion Brendon Watson There is much debate about two stroke and four stroke engines which is causing a great deal of confusion. Here are the facts.
A scooter engine works by drawing air and fuel into a combustion chamber and igniting this mix with a spark creating an explosion which forces a piston to move up and down inside a cylinder and then expelling the spent gasses out through an exhaust, each movement is called a stroke. The names two and four stroke relates to the number of strokes per spark/explosion, so a two stroke engine moves the piston up and down once giving it two strokes per spark and a four stroke moves the piston up and down twice giving it four strokes per spark.
Two Strokes
A two stroke (the top end is pictured on the right) engine is simple in design making it inexpensive to maintain and repair, compact and low weight give them excellent power to size ratio's making them a suitable choice in applications where its advantages can be exploited (such as scooters).
Apart from the strokes per spark the engines also differ in the way the air/fuel mixture is delivered into the combustion chamber. In a two stroke engine as the piston moves air and fuel is delivered through a port (Hole) on the side of the engine into the combustion chamber and is igited by a spark, with the piston moving the other way the spent gasses are expelled through an exhaust port on the side of the engines barrel into your exhaust and into the atmosphere.
So what the hell does that mean?. With less moving parts the engine can be made small, light and the explosion every 2 strokes gives them good power. In a 50cc scooter you get a smaller and more powerful engine meaning a lighter and better performing scooter.
The air and fuel mix that goes in a two stroke has a dual purpose, to create the explosion and to lubricate the internals which is why they don't contain oil in the engine like your car. In a two stroke engine you have fuel in the fuel tank, oil in the oil tank and they are automatically mixed. In Scooter days long gone and in lawn mowing style two strokes there was only a fuel tank meaning you had to manually add the correct amount of oil to the correct amount of fuel (Known as pre mix). This is an absolute pain in the backside and if you got the pre mix wrong you could easily blow an engine. This has unjustly given the two stroke engine a bad name and is being exploited by marketing promoting 4 strokes.
Keep in mind that fuel and oil is mixing to lubricate your engine and that good lubrication is the key to long engine life. The oil you run is critical and it is important that you use the same good quality oil all the time. The petrol you run is also critical in 2 strokes, normal 91 octane unleaded fuel is the go, preferrably from the same station/chain, no ethanol, no 98 octane specials. Why? because some additives and two stroke oil don't mix (pun intended) and will cause all sorts of problems. That's the secret of 2 stroke eternal life
Four Strokes
Quiet, efficient, reliable and smooth. Four stroke engines (top end pictured left) contain oil in the engine and their own internal lubrication system which is more efficient and reliable than that of a two stroke. The oil is subjected to extreme heat, friction and will become contaminated with impurities from the combustion process. Over time the oil in the engine of a four stroke will break down no longer properly lubricating the engine and dispersing contaminates through the engines internals. It is critical that the oil be replaced at the service intervals, if you are are hard on your scoot or live in a warm climate change it more often. One litre of full synthetic motorcycle oil can be had for $15-$20 so its not a big expense. Oil changes are the secret to four stroke eternal life.
In a four stroke engine the air fuel/ mixture enters and exits the combustion chamber by valves that open and close. This means more complexity, more weight and higher servicing costs. The positive is that four stroke engines with their superior lubrication will outlast a two stroke, combine that with lower fuel consumption and the running cost of a four stroke is less.
From a users point of view a four stroke is an easy engine to own, you can use any unleaded fuel, they produce power from low revs making them easy to ride, most people prefer the sound of the four stroke and they don't foul spark plugs meaning they are more likely to start first time every time.
So which one for me?
In the 50cc class the two strokes are still the scooter of choice, the accelleration is acceptable and their lower weight contributes to a lighter and better balanced scooter. If you are even remotely considering derestricting your 50 then rule out 4 strokes as the return for the investment is poor, 2 strokes on the other hand become even more powerful, more fuel efficent and even greener when properly de restricted. The only time to consider a four stroke is if you are under 60kg and do not ever intend to go above 60km/h
Over 100cc's and for most people its four stoke all the way, the engines are big enough to produce sufficient power and the reliability, the power delivery and smoothness also come into play. Change the oil regularly and the 4 stroke engine will easily outlast its 2 stroke rival.
2 Strokes will be outlawed
This is my favourite one. Most 50's on the market are 2 stroke and the latest batch have met with Euro 2 Emmssions. New models are appearing all the time with both two and four stroke engines. The fact is technology will develop (Catalytic converters)) to meet ever demanding emmission standards with changes applying to new vehicles so the government aren't going to come to your house and take your 2 stroke scooter off you for being an environmental vandal .
published 1/02/2007 |