Monday, 20 July 2015

Two Stroke Vs. Four Stroke Motorcycle Engines

The difference between the two stroke and four stroke internal combustion engines have tormented riders for years now, as all sorts of contradictions keep surfacing, leaving most of them still in doubt. This is due to the fact that each one has its own supporters who get defensive and are unable to keep their objectivity. In fact, all they have to do is understand how each of these two engines work and draw their own conclusions.

There are four sequences that need to be cleared out for that matter: intake, compression, power and exhaust. As you might have guessed, there are a few mechanical differences between the two, which we will try to point out next.

Four Stroke Engine

 During the intake stroke, the piston goes down from the top of the cylinder to the bottom, reducing the pressure inside the cylinder. It then draws a mixture of fuel and air into the cylinder through the intake port, ready for the compression stroke.

With both intake and exhaust valves closed, the piston goes back up to the top of the cylinder compressing the fuel-air mixture. This is what happens during the compression stroke.

The compressed air–fuel mixture is then ignited by a spark. The pressure from the fuel-air mixture combustion drives the piston back down with humongous force, keeping the crankshaft turning. This is the power stroke phase, which is the main source of the engine's torque and power.

Finally, during the exhaust stroke, the piston again goes up and pushes the burned gas from the cylinder out the exhaust valve. Another thing worth mentioning is that the spark plug only fires once every two revolutions.

Two Stroke Engine

 Unlike the four stroke one, a two stroke engine has three strokes combined into one action, meaning the intake and exhaust are both integrated into the compression movement of the piston, therefore eliminating the need for valves. This is thanks to an inlet and exhaust port integrated into the wall of the combustion chamber.

As the piston goes down from combustion, spent gasses are allowed to exit the chamber through the exhaust port. The air–fuel mixture is drawn through an inlet located lower in the chamber. As the piston rises again, it blocks off the inlet and exhaust ports, compressing the gasses at the top of the chamber. The spark plug fires and the process starts over. The engine fires on every revolution.

Let the fight begin!

A two stroke engine can produce twice the amount of power (and makes twice as much noise) than a four stroke engine of the same size. This is because it fires once every revolution, giving it twice the power of a four stroke, which only fires once every other revolution. Significantly, it also has a higher weight-to-power ratio because it is much lighter.

Two stroke engines are simpler and cheaper to manufacture compared to four stroke engines because of their simpler design. Four stroke engines are longer lasting than two stroke engines that don't have a dedicated lubricating system. However, the spark plugs in a two stroke engine last longer than those in a four stroke engine.

Four stroke engines are more fuel efficient and environmentally friendly when compared to two stroke engines that also create an unpleasant smell. Two stroke engines are responsible for much more pollution due to the combustion of oil.

A Common List of Advantages and Disadvantages
Advantages of 2 Stroke Engines:
  • Two-stroke engines do not have valves, simplifying their construction.
  • Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
  • Two-stroke engines are lighter, and cost less to manufacture.
  • Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution. 

Disadvantages of 2 Stroke Engines:
  • Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
  • Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
  • Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
  • Two-stroke engines produce more pollution.
  1. The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
  2.   Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.

Clearing the Air

Michael Harrison from the DeepScience BIGENZ team has this to say:
Most of what is written on advantages and disadvantages of 2 strokes Vs 4 strokes is not actually correct. 

Take for example the lubrication issue of 2 stroke engines, sure small chainsaw engines may have the oil mixed with the fuel but this is not a direct result of the engine being a 2 stroke, this is just a result of someone designing a very simple engine. look at any large Caterpillar, or Detroit 2 stroke they have conventional oil sumps, oil pumps and full pressure fed lubrication systems and they are 2 stroke! 

Also, the argument about valves of 4 strokes versus the reeds and ports of 2 strokes is also incorrect. Sure some simple 2 strokes may use very primative systems to achieve the conrol of fuel/air mixture into the engine and exhaust out of the engine but again this is not a function of them being 2 stroke! I've worked on 2 stroke engines that feature poppet valves in the head (like a standard 4 stroke) - but they are definately 2 stroke - it's just that engines like this are not so much in the public eye - next time an ocean liner (ship) pulls into port check out its 2 stroke, turbo charged, direct injected diesel engine!

Finally, the arguments of simplicity, weight, power to weight, and cost of manufacturing are not a function as such of 2 stroke versus 4 stroke engines. The mistake of most of these commentaries is that they are comparing a simple chainsaw 2 stroke engine with a complex 4 stroke engine from a automobile - not a very fair comparision. 

As far as the exhaust emmisions of 2 strokes - check out the Surrich/Orbital 2 stroke design that Mercury outboards are using - this is as clean burning as any 4 stroke.
The ONLY correct comparison of 2 strokes with 4 strokes is that a 2 stroke can (in theory) produce twice the power of a 4 stroke for the same sized engine and the same revs.

So Which is Better?

At the end of the day the winner is probably going to be the one that has had more money and technology spent on it. In these days of quick and cheap international production schedules you can't take it for granted that the 4 stroke will be better. So for your particular application, line up the options and make a decision based on what's available, not based on lists that miss the key points of difference.