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Hey Buick Deere...A question for you...

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BushogPapa

05-16-2008 11:09:22

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Hey, B&D...ever consider the actual compression ratio that a typical car engine operates on..?? I don't mean the mechanical compression ratio. I mean the actual compression ratio when you figure in the VACUUM level restricting the fuel/air Volume to the cylinder... At 20" vacuum in the intake, I doubt an engine is operating at a true ratio of over 4:1 !!! I wonder what would result from a variable compression to maintain optimum Combustion Pressure, albeit at a huge reduction in volume..?? What is YOUR Take on this idea..?? Ron.

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BushogPapa

05-17-2008 22:02:58

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

Let me put this another way..

If you use a compression gage on an engine and have a RAG stuffed in the intake, blocking it...won't the gage read LOWER..?? Ofcourse it would, and that is what is happening all the time you drive at anything less than WOT..! What I am asking is that it seems to me that an engine would be more "fuel-efficient" if it could vary the compression to maintain the maximum combustion Pressures that the fuel can allow at all times...even at Idle. This would need variable compression, coupled to the throttle, via a computer.. Does this sound realistic..?? I guess I'll go back to the basement and mix more Model Diesel fuel...Ether, Kersene, Castor..anyone see where I put the Amyl Nitrate..??

Ron..!!

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dr sportster

05-16-2008 14:48:55

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

One way to reduce effective compression is to use a cam with a later intake valve closing time.This would be like in an attempt to make a high comp engine start easier.However generally, the engine power band or when it "comes on" will be higher in the RPM range.You sound like you dont own a compression gauge because 4:1 is just not right.Actual compression is approximatly what the guage reads.

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Mathias NY

05-16-2008 13:41:58

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

Compression ratio is just that, a ratio. This is probably the mechanical ratio you were refering to, this will never change.

Assuming you start at 1 atmosphere of pressure, a 4:1 compression ratio would give you 4 atmospheres of pressure. Yes, I suppose there could be less than 1 atmosphere at the start, but it's probably insignificant and quite consistant.

More air (via turbo or super charging) will increase the pressure that you start with and therefore the increased pressure upon compression. Water injection is a common way to decrease the compressable volume of air in the cylinder and increase the compression pressure.

Somehow I don't think I answered your question, but I'm not really sure what you asked...

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jdemaris

05-16-2008 13:27:52

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

I'm not sure I understand what you mean by "actual compression ratio." A typical auto engine - especially going back a few years has a volumetric efficiency of 60-70%. That applies to gas, and also to diesels with a wide-open intake. Most naturally aspirated diesels only run a 60% volumetric efficiency. That means the engines can naturally draw in around 60-70% of their cubic inch displacment without help from a supercharger of some type - abeit belt, gear, or exhaust driven. The mechanical compression ratio is simply the ratio for the cubic inch displacement at it's largest (bottom of stroke) to its smallest (top of stroke). Add a supercharger that forces air in - and the "effective" compression ratio comes into play - which can be quite a bit higher.

As far as I know (perhaps not far enough?) - a engine with an 8-1 mechanical compression ratio is just that - and NOT less regardless of intake restrictions.

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buickanddeere

05-16-2008 13:21:27

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

Jon beat me to it.
The concept is older than any of us.
The variable valve timing engines are pretty clever and effective. Control of overlap, combustion chamber filling, residual exhaust gas amounts and prevention of raw air/fuel mixture making it out the exhaust. It's the way to have both an Atkinson Cycle Engine and an Otto Cycle engine.
For single speed, steady load engines. Engineers have tweaked the compression ratio and intake timing into Atkinson Cycle for years. Getting Atkinson Cycle to work on a variable speed, variable load engine requires the speed and accuracy of computer control.
I can't recall if it was Crower, Manley or who the west coast race engine parts builder was in the 1970's or 1980's. They drove a 350cu" delivery truck with 14.5 to 1 mechanical compression using modified 327 pistons. And a long duration intake cam lobe profile. There is also work on stratified charge engines where the gasser doesn't use the throttle at cruise. The combustion chamber takes a full charge of air. Then a mist of fuel is kept very close to the sparkplug. If this mist of fuel was spread around the combustion chamber the ratio would be too low to burn.
Pumping losses are reduced as it heat losses to combustion chamber walls.
Hydrocarbon emissions are down as well as the fuel droplets don't "touch" the combustion chamber walls. Fuel that touches or is very close to combustion chamber surfaces either doesn't burn or only partly burns. One of the reasons an engine fully up to temp is more efficient than when cold. Also why an engine is better off running with a 195F stat instead of a 160F stat or non at all.
True compression ratio maybe even lower at 2 to 1 at part throttle/idle.
The lower combustion chamber pressure and density is why flame travel is slower. This is why vacuum spark advance was introduced to increase part throttle efficiency.
There is some tinkering going on with six cycle gasoline and diesel engine with a much reduced cooling system. Water is injected and flashes to steam after the exhaust stroke. For another "power stroke".

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MarkB_MI

05-16-2008 12:58:07

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

You don't need to to anything fancy like varying the compression ratio. Simply operate the engine as close as possible to wide-open-throttle at all times, which eliminates the throttling losses. Contrary to popular belief, high vacuum (aka low manifold air pressure) does not actually translate to high efficiency. This is the reason a number of strategies to improve fuel efficiency work. For example:

Smaller engines in the same vehicle tend to get better gas mileage, because they operate closer to WOT.

Taller final drive ratios get better mileage, because they require higher engine torque (higher MAP) under the same conditions than a shorter (numerically higher) axle ratio.

The engines on hybrid vehicles spend much of their time at wide open throttle, storing energy for future use.

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Gerald J.

05-16-2008 15:11:03

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Re: Hey Buick Deere...A question for you... Prius engine in reply to MarkB_MI, 05-16-2008 12:58:07

And the Prius engine runs a modified cycle. It's mechanical compresion ratio is about 12:1 but they hold the intake valve open for half the compression stroke so the real compression ratio is only 6:1 but the 12:1 expansion ratio makes the engine extract more power from each combustion. And so the Prius running on the engine can still do 50 to 60 mpg on the highway.

Gerald J.

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Jon Hagen

05-16-2008 12:22:43

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

yes, maintaining more efficient cylinder pressure at light throttle is not a new idea.

One place where it is in use today is in the Atkinson 5 cycle engine used if the Ford Escape hybrid.

Link

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Bob Farrell

05-16-2008 11:59:38

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Re: Hey Buick Deere...A question for you... in reply to BushogPapa, 05-16-2008 11:09:22

I really think you are trying to say something or ask a question. Which is it? and what are you saying? Please explain a little more so this redneck can understand. Thanks! bob f.

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