# Restricted internal combustion engines

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As part of an engineering course, I have been presented with a list of topics to research. It seems that my next few tutorial sessions will deal with internal combustion engines quite deeply. This is a bit of a culture shock to me.

The list is presented as a series of questions. I've managed to find information on most of them, but one has totally stumped me.

"A four-stroke motorcycle engine has been restricted by having its exhaust pipe partially blocked close to the outlet from the engine. In general terms, how would you expect the torque-speed relationship of this engine to differ from that of an unmodified engine, and from that of an equivalent two-stroke engine?"

Thanks in anticipation.

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Draw the cyle diagrams for the unrestricted 4 stroke & the 2 stroke.

Think about the power stoke (at constant speed)

Think about how the restriction would modify the 4 stroke diagram

Compare the modified 4 stroke to the 4 stoke & the 2 stroke

Now extrapolate to include the effects of speed.

These types of exercises will be much more beneficial to your learning if you can talk about the real time with a study group & a handy white board.

cheers Bob

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"Enzo Matrix" wrote in news: snipped-for-privacy@giganews.com:

Talk to your local mechanics about cars with mostly plugged cat converters.

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Here is an arm-waiving explanation. Visualize the power stroke of a four stroke. The pressure developed on the piston depends on the amount of fuel avaiable - given a reasonable mix with air. This power stroke delivers a certain volume of exhaust gas to the exhaust manifold. Higher throttle openings or higher revs, both deliver more exhaust. A restricted gas flow creates a back pressure proportional to the flow rate. On can reasonably suppose that high revs at high throttle which produces the most back pressure at the exhaust valve, chokes off the flow of fresh mixture. On can thereby suppose that at low revs, a good charge can provide high torque. At higher revs, torque diminishes per power stroke, so HP flattens while torque declines with increasing speed.

Brian Whatcott Altus OK

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Many thanks to everyone who replied. Your replies have been most helpful and are very much appreciated.

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Why exactly do you wear cheese? Some strange sort of fetish?

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LOL It's a quote from an episode of "Buffy the Vampire Slayer". The line used in BtVS was also a reference to a line from Leonardo di Caprio in "The Man In The Iron Mask".

"I wear the mask. It does not wear me."

But... I am *very* partial to a nice bit of cheese every now and again.

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Enzo-

cheers Bob

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As far as I can see, the restricted engine will operate relatively normally at lower speeds. However, once the exhaust volume exceeds the flow rate of the restricted exhaust port, the cylinder will not be cleared of exhaust gases during that cycle. The next charge will be composed of smaller volumes of air and fuel as the cylinder already contains a certain amount of burned gases. Therefore at higher speeds, there will be less torque available.

In effect, the peak torque of the restricted engine will be available at a lower rpm in comparison to the unrestricted engine. Once the exhaust volume exceeds the flow rate, that torque will drop dramatically. I'm also of the opinion that the peak torque of the restricted engine will not necessarily be of the same magnitude of the unrestricted engine.

Am I working along the right lines?

You mentioned that the excercise would be beneficial in a real time discussion with a white board. I think that's *exactly* what our tutor has in mind, and why the questions on the preparatin paper were couched in such vague terms.

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Enzo-

Your analysis sounds good to me so far. It's been years since I've done any gas engine cycle analysis.

Here is something to consider as the opposite to your situation....an exhaust manifold expansion chamber.

Also how did the restricted 4 cycle compare to the 2 cycle?

Your tutor is correct in posing vague questions...that is what often engineering is about.....vague, poorly posed questions that really make you think to solve them

cheers Bob

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