2 Stroke Vs 4 Stroke – you decide.
For many years two stroke engines dominated the snowmobile industry. Now every major manufacturer carries four stroke snowmobiles. Only Yamaha is one hundred percent four stroke.
I remember when I was a teenager in the late seventies and there were rumours that Honda was developing a four stroke snowmobile. Unfortunately the project was cancelled. I suspect that the four stroke technology of that era was not sufficient to compete with the two strokes. Four strokes then had two valve cylinder heads and moderate compression in the range of 8-9 to one. The newer four strokes now have four or five valves per cylinder and compression ratios above twelve to one resulting in significant improvements in horsepower and torque. Imagine taking twelve ounces of air/fuel mixture and compressing it into a one ounce space. The molecules are packed so tightly that it burns more efficiently. This results in more power, better fuel mileage and less emissions.
The two stroke engines have come a long way too. Engine designs have improved as well. I remember when the Tillison carburetor was replaced by the Mukuni carb resulting in a 30-35 % increase in fuel mileage. It’s kind of mind blowing to get that kind of improvement by changing a carburetor. Most of the larger snowmobiles now a days are fuel injected. Ski-doo has gone a bit further with direct injection. It’s a big step ahead in efficiency. Basically the fuel is injected down from the top of the cylinder head only after the piston blocks the exhaust port, resulting in very little if any unburned fuel going out the exhaust.
The power valve was a great improvement as well. It allowed the use of a larger exhaust port (more power) and had a guillotine valve that could open and close in effect creating smaller exhaust port at low/moderate speed and opening up fully at higher RPM. It saved fuel and made more power.
I’m going to describe the attributes of both engines .
The two stroke engine is cheaper to build than a four stroke. The four stroke engine is more expensive, but will last significantly longer than a two stroke.
The two stroke engine is lighter and simpler than a four stoke. The four stroke engine is heavier, has many more parts resulting in more friction.
Two strokes are easier to start (pull or electric). A four stroke requires a heavier duty starting system because of higher compression.
Two strokes have a power stroke every revolution. Four strokes have a power stroke every second revolution.
Two strokes have a narrow useable power band (3-3500 rpm). Four strokes have a very broad power band (6-7000 rpm).
When a two stroke intakes both intake and exhaust ports are open resulting in unburned fuel going out the exhaust. This is mitigated partially by the power valve. This is not an issue for four strokes.
Unmodified two strokes are generally louder than four strokes.
A two stroke is not able to force out all the burned gasses at part throttle because there is only a small amount of air entering the motor, resulting in incomplete combustion. The four strokes don’t have that problem.
Two strokes produce more heat requiring a heavier cooling system than a four stroke.
Two strokes will produce about 30% more power than an equivalent displacement four stroke. A four stroke is more efficient than the two stroke because of higher compression ratio. (four strokes have compression ratio of twelve+ to one verses eight or nine to one for a two stroke) This year’s Yamaha YZ 250 motocross bike has a compression ratio of 13.5 to 1. That’s close to the low end of diesel engine compression levels.
Two stroke engines have a lower center of gravity. The four stroke engine is higher. Although most of the engines are laid back, the center of gravity is higher because of weight of the cylinder head.
Most fuel injected snowmobiles will probably run at high lean angles maybe even upside down. This isn’t a problem for two stroke engines. (ie. Chainsaws). However the four stroke engine has the oil pump in the bottom of the engine that feeds oil under pressure (30-60 psi) to all the bearings in the motor. If the machine is at high lean angles or upside down oil flow to the bearings maybe be interrupted resulting in engine damage. Four stroke owners should be aware of this.
This article by Paul Barron was originally printed in the Nov 2014 Edition of Sledworthy Magazine.