View Full Version : Two stroke Engines

20th Jun 2002, 13:09
How do two stroke engines work, which are very popular for their high power to weight ratio in microlight planes ???

Sounds like a silly question ?

OK, a little more detailed :

I am running a two stroke with reduced power, lets say at 15 inch hg manifold pressure. The Piston has reached its position nearest to the crankshaft and both īvalvesī are open, so the crankcase housing containing fresh air fuel mixture, the cylinder and the exhaus pipe and muffler are connected to each other.
Now I have fresh air fuel mixture with 15 in hg in the crankcase and air with 30 in hg at the outermost end of the exhaust.
What makes the exhaust go to the higher pressure and makes the mixture comming into the cylinder (from low to high pressure) ???? How do I force all the exhaust gas to leave the cylinder (because if too much remains, the mixture will not burn if ignited)

Someone able to explain this kind of magic ?

Cornish Jack
20th Jun 2002, 19:08
Strangely enough, if you want the REAL info on two-stroke operation you need to talk to a MODEL aircraft flyer - particularly one of the pylon racing brigade. What they don't know about porting, pressure patterns and the effect of getting the engine 'on the pipe' with tuned pipe set-ups, isn't worth knowing. It's a bit of a black art and for sheer adrenaline-producing, hair-on-the-back-of-the-neck-raising noise you should hear the sound when a 'piped' 10cc motor 'goes on the pipe'. Awesome!
To return to your original question, from a strictly marginally involved perspective, I understand that it's essentially to do with pressure patterns and their interactions. With properly engineered porting and timing the outputs can be spectacular. As far as I know, the record power output per cc swept volume still rests with a two-stroke - Susuki - if memory serves me correctly....but go find a modeller!! :)

20th Jun 2002, 21:15
The first point to note is that 2 strokes do not use conventional inlet and exhaust valves. They use ports (holes) in the side of the cylinder, which are covered and uncovered by the motion of the piston.

The inlet manifold is connected to the crank case and contains a non return (reed) valve which prevents flow reversals. The crank case is also connected to the inlet port.

As the piston moves up in the compression stroke it closes off both of the ports, and so compresses the mixture in the cylinder. This upward motion also draws mixture into the crank case. The mixture in the cylinder is fired close to top dead centre and the expanding gasses push the piston down.

As the piston moves down it tends to pump the mixture out of the crank case. But the reed valve prevents this reverse flow, so the mixture becomes compressed in the crank case.

When the piston is close to bottom dead centre it uncovers the exhaust port and the pressure in the cylinder causes the exhaust gas to start flowing out. A fraction of a second later, the piston uncovers the inlet port, alllowing the pressurised mixture in th ecrank case to flow into the cylinder. The piston crown is shaped such that the incoming mixture helps in pushing out the exhaust gas.

The piston then starts to rise, and the process is repeated.

20th Jun 2002, 21:59
A model two-stroke engine is probably the most simple thing to look at in order to understand the principle of operation. The fuel enters thru the carburator and into the crankshaft ! which is hollow. from there it is sucked into the crankcase by the vacuum created from the piston moving up. When the fuel has reached the crankcase the piston is around the top of it's stroke and is forced downwards again from the expantion of burning fuel. As written earlier the two-stroke engine has no valves, but instead ports in a liner around the piston. This liner has from the intake-ports a room between the cylinder and liner which goes down into the crankcase where the fuel is and as there is positive pressure from the combustion this pressure is relieved thru the exhaust-port and out into the muffler or pipe or whatever. This port is located slightly higher up than the intake-port for the purpose of relieving pressure before both ports are open and fuel can enter. (The ports are opened and closed solely based on the position of the piston, by it's movment.) As this pressure gets less than the pressure the piston creates in the crankcase on it's way down, the fuel-air mixture is forced thru the intake-ports into the cylinder on top of the piston and the cycle starts again with fresh fuel/air mixture being compressed. The fuel is ignited by the plug which in model-engines is a plug electrically heated at start, and is kept glowing from the continous heat from the combustion.

Fine piece of mechanics, eh?

Here is a link which shows the principle mechanics of the two-stroke engine: http://www.buckeye-illinois.com/2%20stroke%20principle.htm

21st Jun 2002, 00:03
There are two types of "wave" occurring in a 2 stroke engine exhaust.

The first is a pressure wave caused by the "slug" of exhaust gas entering the exhaust pipe. This gas has inertia and tends to keep on going. Behind it tends to occur a low pressure.

The second "wave" is a shock wave (positive pressure) caused by the sudden opening of the exhaust port. This wave doesn't have inertia and can easily be reflected. As the wave travels outwards and leaves the pipe end, a negative pressure reflection occurs at the outer end of the pipe.

The skill of tuning these engines for absolute maximum power comes from the shape and length of the pipe and as already been mentioned, from the shape and "timing" of the ports. Timing the ports is achieved by their position in height on the side of the cylinder. Getting these two just right can cause a double negative wave to "suck" the remaining exhaust gases out of the cylinder and allow a very full or even "supercharged" fresh charge of mixture to enter the cylinder.

A characteristic of 2 strokes is that they tend to have quite a narrow power band. A particular exhaust may work brilliantly at one engine speed but not at all well at lower or higher RPMs. "Coming on the pipe" can be likened to a musical wind instrument player suddenly hitting the perfect note. The intake reed valve mentioned earlier is actually quite a recent innovation, as are ancilliary systems which vary intake and exhaust characteristics to make an engine more flexible and economical with low emissions. It was the difficulty of keeping emissions low enough to comply with modern legislation that almost killed the 2 stroke motorcycle industry over the past few years. The fresh charge tends to mix with the previous stroke's exhaust gases.

Some older motorcycle engines would run backwards if the ignition timing was incorrectly set, very interesting for the rider and highly amusing for onlookers!

The next logical step is 2 stroke direct injected diesels. They can give the best of the 2 stroke petrol engine (high power and light weight) with the economy of a diesel engine but only pump air on the induction stroke so the emissions can be much more strictly controlled. I am certain we are going to see a lot of cars (and hopefully aircraft too) running these engines in the next few years. Diesels of course don't suffer from carb icing - there isn't one - and they don't require vapourisation of the fuel in the manifold.

Last week I ported up my son's R/C car glowpug engine for him. The increase in power is amazing; so much so that the car will wheelie and flip completely over backwards if given full throttle too quickly. The downside is that he's told me it is now illegal for the class of racing that he was going to run it in!

21st Jun 2002, 10:06
The basic advantage of a two-stroke is it fires twice as a often as a four stroke for the same revs. Add in the savings in weight due to no moving valves and you have a potent formula.

Unfortunately in most situations it is not power per weight, or power per litre of capacity that count - It's power per litre of fuel, and two strokes are poor in this respect because of the mixing of inlet and exhaust flows.