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The Background::
Superchargers and turbochargers create pressure ratios, this is the ratio of the compressor's output pressure to the compressor's intake pressure. For example a car with 14psi of boost at an atmosheric pressure of 14psi will see an intake manifold pressure of 28psi absolute. So the
pressure ratio is 2. If you have 2 SCs producing pressure ratios of 3 in series (output of the first feeding the intake of the second) you end up with an overall pressure ratio of 9.
4 stroke engines::
Most naturally aspirated engines have a compression ratio of 9:1 - 12:1. This is the pressure ratio of the combustion chamber at top dead center (minus the pressure added by the combustion already taking place). So the goal is to replicate this enviorment without the extra 2 strokes (intake and compression).
Design::
The SCs would have to be lysholm/twin screw type since those are the only positive displacement SCs that can effiecently produce pressure ratio's over 2. I don't know how a similar design with turbos would idle, but a multi turbo system would be prefered as it would be more compact.
The engine would be more like a 4 stroke then a 2 stroke. The difference being that the camshaft would turn at the same speed as the crank.
As the piston moves towards BDC the exhaust valve would open. At some point during the piston's upwards travel the intake valve would open. The high pressure of the intake air should quickly scaverge the remaining ehaust gases. The exhaust valve will close shortly after the intake valve opens. The spark plug fires, intake valve closes and the cycle starts over.
The only problem is supplying fuel. You probably wouldn't want to inject fuel before the SCs as I don't know what effect liquids would have on the SCs rotors. However to inject after the SCs would require a ton of pressure. The intake manifold would be seeing pressures well over 150psi... this would require fuel pressures to be over 200psi!!!!
The effect::
By replicating the conditions of a 4 stroke engine after the compression stroke the 2 stroke should produce the same amount of power and be just as effiecent as the 4 stroke per powerstroke. The advantage is you get 2x the powerstrokes per engine rpm, so 2x the power. A 2.0L I4 would produce the same power as a 4.0L V8.
A performance engine would want a large combustion chamber to displacement ratio. A fuel efficent engine would have a small ratio getting the most out of every powerstroke.
Something you might find interesting
http://www.siscom.net/~louisekramer/
[half, Oct 04 2004]
two stroke diesel
http://auto.howstuf...sel-two-stroke1.htm
[davidcreede, Oct 04 2004]
Lime Engine
http://www.limengine.com/
A little more refined... don't know how close to production [SublimeGTP, Oct 04 2004]
Direct Injection from Mitsubishi
http://www.mitsubis...logy/GDI/page1.html
[madness, Oct 04 2004]
[link]
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You're in the wrong darn website. Take this to shouldexist.com and see what the brainiacs have to say. Read the teflon slide post before you go, it will make you smile. |
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Am I reading this right? The intake valve would open during the middle of the upstroke? How is it the fuel would get into the cylinder--because of tremendous compression generated externally prior to introduction to the cylinder? |
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Does it not make more sense to use ports and a reed valve driven from the crankshaft to control the intake and exhaust timing? The ports would be exposed at all times during piston travel but the down (and up) stream reed valve would control the flows in and out of the port paths. |
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Diesel locomotives do work on this principle, and I've read of plans to do the same thing in automotive engines. I don't know how your idea differs from any of those other ideas. |
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BTW, I had an idea for an engine which would combine the principles of an internal combustion engine, external combustion engine, and jet. The primary feature would be the ability to have a longer combustion-power time than in typical internal-combustion engines. |
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Interesting concept... However, after the exhaust port opens, cylinder pressure goes down to near atmospheric. The intake manifold has very high pressure within. When the intake valve opens, there is a very high pressure ratio across it. As the charge enters the cylinder it is throttled much like a throttle plate in a carburetor. This is a very large efficiency loss. Two reasons why a CI engine (diesel) runs so much more efficiently than an SI engine (gas): 1) the CI engine does not have a throttle plate, and 2) it has a higher compression ratio. Also, as the air is throttled on its way into the cylinder, the temperature of the air goes up, thus the density down. You will not get as much charge into the cylinder as a traditional 4-stroker. You will not get twice the power as a 4-stroker. Don't forget about the power required to turn the SC. |
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First, I believe the current limiting factors in turbo technology are spindle speed and size. |
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A larger sized turbo must spin slower and cannot produce as large a pressure differential. It can, however, push more air volume at that lower pressure. I know there is a lot of turbine design involved in the pressure to volume ratio...I'm ignoring that. |
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A smaller turbo can spin faster and produce a higher boost, but cannot move as much air. |
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While a 200psi boost at the engine would be great, you must also consider that your engine would explode when this 200psi is multiplied many fold by the gasoline in the cylinder combusting. |
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Last but not least, the airflow through a supercharger/blower (the huge suckers on muscle cars) is pretty high in the first place...and they only produce something like a 40 psi boost at max RPMs (eat a lot of power, too, but make up for it!). So, you'd need a hell of a filtration system to handle the airflow for a 200psi boost system...unless you really want to aspirate dust, etc into your engine. |
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2 stokes diesel naval engines work in similar way |
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This is similar to a two stroke diesel (see link), but I don't see any reason why one couldn't make a gasoline version work. I was heart broken when I discovered such things already existed. |
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I cant belive no one knows about this, Its called a miller cycle motor and they are acctully in a car today...The mazda millenia S, not the standard one, the standard one comes with a 2.5 but the faster one comes with a 2.3 miller cycle supercharged and intercooled motor. Check it out. I allways wanted to do this to other cars too by just making the cam to crank spin at 1:1 or double the lobes on the cam itself. |
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I guess this is a 2 stroke diesel with a spark plug... damn... thought I had an original idea. |
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LexusTech,
This is nothing like a miller cycle engine. A miller cycle is a 4 stroke engine with an elogated intake duration (into the compression stroke). |
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Yeah, this is a blown two stroke diesel, one of the classic engine designs, lost of big huge ones around because in a huge engine, the gains in efficiency are just too big to ignore. You came up with such a great idea, they are already in widespread use! |
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Why they're used for diesels not gas engines is because some of the intake charge gets lost out the exhaust. If it's just air, like in a diesel, that's fine, but if it's got gasoline in it you increase fuel consumption and emissions. |
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Remember, those big Roots blowers they use on dragsters were originially designed for diesels. |
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The Miller cycle is different. It keeps the intake open during part of the compression stroke to reduce the engine's actual compression ratio, while still keeping a large 'expansion ratio' (to coin a phrase) after ignition, so as to maximize the efficiency of a supercharged engine. |
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It looks like someone else had the same idea...
They even have a better idea for the intake valves, no huge pressure differential and mechanically simple. |
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The strokes eliminated (or provided externally) would be induction and exhaust. The new charge forced into the cylinder is also required to displace the spent charge after the combustion stroke. So some positive displacement compression will be provided by the engine. This is key to the operation of the 2-stroke diesel. |
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The engine is a two stroke diesel running at compression ratios two low to ensure combustion. It will require direct injection to prevent detonnation and ensure unburnt fuel is not forced into the exhaust. It will also require a spark plug to ensure combustion, ie two holes in the head one for a spark plug and the other for a diesel style fuel injector. |
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It will be lighter and more powerful than a similar sized diesel engine because of the type of fuel used. Diesels are heavy because of the high compression ratio and not as powerful because of the lower grade fuel. (They are more efficient but that is not the purpose of this engine.) |
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A small roots type (positive displacement) super charger will ensure good external compression for starting, ie the roots type blower is simply a piston based air compressor. A small roots compressor will produce the right volume of compressed air to feed the engine at low rpm. |
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Remembering the miller cycle a screw type turbo charger can be used at higher rpms since this compression is free and the screw type compressor is more suited at high rpms. (A roots type super charger will simply mean a 2L engine will require a 2L piston compressor.) |
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So given that the heads can be modified to accomodate an injector and spark plug and a supercharger and turbocharger can be mounted to provide compression, how should the values operate. |
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For this engine to provide a theoretical doubling of HP. It must operate at least the same rpms of a similar sized 4 stroke. You might expect the red line of this engine to be half that of the 4-stroke since the values need to open and close twice as often, ie no increase in power. |
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The values on this engine will operate at twice the rate but at the same speed (distance/second) of a similar sized 4 stroke. So I would suspect that the cams and belt will need maintenance twice as often but that they will operate at the require rpms. |
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