Would this work??? 2 stroke improvement?

Matt Wilson /

I've been reading a little bit about 2 stroke motors in a book I checked out from the Library. Its basically a text book, and its pretty dry...

It brought up the fact that 2 strokes are dirty, and the pollution comes from 2 sources, unburned gas, and burned oil.

The book said that in an effort to increase cleanliness, many 2 strokes use oil injection. Oil injection systems burn considerably less oil than premix, and in some cases can approach 200:1 gas to oil. It also mentioned that 200:1 is very close to the amount of oil that a 4 stroke burns.

The other source of pollution is the unburned gas that escapes out the exhaust port, before it closes. My idea is to use a reed valve in the exhaust.

A stiff reed valve over the exhaust port would not allow any air gas mix to escape while the intake is open, because there is very little pressure. However, after combustion there is a lot of pressure, and the reed valve would open and allow the exhaust to flow out.

If this worked it would increase gas mileage, increase performance, and decrease pollution.

I doubt that I'm smarter than every engineer at honda suzuki, yamaha, and kawasaki combined, so there must be some reason this wouldn't work. I dont know... what do you think?

Matt

Re: Would this work??? 2 stroke improvement?

Go to the hero puch web site. Their new moped has an exhaust valve.

Several reasons

... why that won't work .. (or work as well)

First is materials and heat.

... reed valve blocks use a rubber coating to maintain a good seal for the reeds .. (at least 'normal' intake ones do) .. the rubber would melt.

.. I don't know that you could find a metal alloy that would retain the needed flexibility that reeds have to have after being exposed to the very high heat they would get ... like 1200 degrees F.

Too restrictive in the exhaust port.

The exhaust port is a large wide open high speed flow area... to get the equivalent flow area out of a reed block it would have to be real big.

Most important though ! .... Reeds are a one way valve ... they would totally eliminate the ram effect of the sound waves returning before exhaust port closes and cramming mixture thats in the pipe - back into the cyl ... that makes two strokes what they are.

Remove that effect from a 2 stroke and it might as well be a low performance low HP lawnmower engine ... putt putt putt putt

(mind you .. 50cc restristed to 2hp are low performance anyway - because of government rules .. but who wants that?)

Modern 2 strokes all use mechanical 'power valves' in their exhaust ports .. a big improvement .. they allow both good low RPM performance and good high RPM performance.

(they are essentially a variable height roof for the ex. port ... the roof comes down at low RPM and goes up at higher RPM)

Re: Would this work??? 2 stroke improvement?

Hi Matt.

Engine tuners try to design the exhaust system such that there is a slight back-pressure from the exhaust working against the fresh fuel/air mixture trying to exit the exhaust port. This is essentially a dynamic reed valve. Some of the fuel vapors that were on their way out the exhaust port get pushed back into the cylinder. The effect is difficult to achieve over a large range of rpm, but one can achieve it at a certain range of rpm. If there is too much back-pressure, then the fresh fuel/air gets stifled by the exhaust gases re-entering the cylinder and power is lost.

Also, at high rpm, there is an effective increase in the compression ratio, because the fresh fuel air mixture has less time to escape out the exhaust port, because the port is open for a shorter duration. This is also a sort of dynamic reed valve. The back-pressure and dynamic compression effects together can cause a power peak at high rpm.

I suppose the reasons are technical for not having an output reed. Unlike an input reed, an exhaust reed would have to withstand high temperature and pressure. If you look at the exhaust pipe on your moped (if it has a chrome pipe), you see the discoloration from the oxidation. The reed would have to be made of nickel and chrome or plated with something that doesn't oxidize at high temperature. Also, it would be difficult to maintain the spring-like nature of a thin metal reed on an exhaust port, because the metal will soften at high temperature (take a steel spring and heat it up to a few hundred C for 15 minutes or more and see how it loses its springiness).

The output valve would have to be more like that of a 4-cycle engine exhaust valve (or perhaps a rotary valve) rather than a reed. If one could make such a valve, then there would not be a need to use exhaust gases from the exhaust pipe to block fresh fuel vapors from exiting the cylinder. It would still be a two-cycle engine, though.

Some engines have a variable size exhaust port, but this is not the same as a reed valve.

Paul

wanna check out a cool engine??

Matt Lorence /

Ever see a pulse jet engine? it takes the idea of back pressure to a whole new extreme. Ever hear of an engine with no moving parts? Yeah, thats a pulse jet too, although they sometimes have a reed valve on the intake for more control of the engine. heres a site. http://www.aardvark.co.nz/pjet/ look around it, if anything, its humorous, to the more inquisitive, its possibly a start for a extrordinarily high speed moped.

Ducati valves

Ducati uses passive intake valves that act like reed valves. Maybe set up some exhaust valves on a two stroke that have weak spring pressure. They would restrict the low speed backpressure for low-end torque, but stay open for high-end horsepower. Maybe two valves with different spring pressures. You would have to add a valve cover for oil. Exhaust would be too hot to want the oil in the carburated intake lube it.

Ducati valves

Ducati uses passive intake valves that act like reed valves. Maybe set up some exhaust valves on a two stroke that have weak spring pressure. They would restrict the low speed backpressure for low-end torque, but stay open for high-end horsepower. Maybe two valves with different spring pressures. You would have to add a valve cover for oil. Exhaust would be too hot to want the oil in the carburated intake lube it.

Re: Ducati valves

Thanks for the link to the pulse jet engine. That captured my attention. That's going to be a project for me, i've already started :)

Re: Ducati valves

actually, Ducati 4-stroke engines use what is known as a 'desmodronic' valve set-up. there are 2 cam lobes per intake valve one opens, one closes the intake (and exhuast) valves. instead of a spring to close the valve, camshaft lobe closing the valve means a pretty much unlimited rpm capability only limited by the reciprocating mass of the bottom end of the engine. It's active. a valve spring is passive.

another thing to consider about the exhaust setup is pipe shape contributes to the 'scavenging' effect you guys are talking about. the taper in the front and back of the pipe contributes to pulling the exhaust out, the slowing the flow (backpressure) at the end of the charge. actually incrasing air flow thru the combustion chamber. a reed valve would probably negate this effect. limiting horsepower.

2-stroke DIESELS are a whole 'nother animal. Detroit Diesel 2-stroke engines have intake ports force fed by a low pressure (4psi average) blower. in the top of the cylinder where the fuel injector is, there are 4 exhaust valves.

Re: Ducati valves

do you understand the words coming out of my mouth?

Re: Would this work??? 2 stroke improvement?

Matt, check this link.

http://www.motorcycle.com/mo/mchonda/exp2_tech.html

Honda is using an exhaust valve similar to a power valve but for the purpose of reducing emissions. It's a 2-stroke that uses a variable height exhaust valve and exhaust gases to auto-ignite the fuel/air mix.

Paul

or...

to save pollution 7 waste of unburned fuel, you could attach a spark plug in your muffler and have an afterburner like a jet engine.... or maybe not...

Geoff

Re: or...

Danko Craig /

right, and we'll all buy lead sleds with dayton wire wheels and have competitions at night..... LOL

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