Talk:Manifold vacuum

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I redesigned the entry, and "the basics" and created appropriate links regarding air pressure. As a gasoline engine expert, I don't agree with everything beyond "the basics". But, I didn't want to stomp on the work of other's, and there is value in all that discusson for whomever wants to read it. Sattyam 17:29, 8 October 2005 (UTC)

I reworked much of the article for tighter organization -- like Sattyam, I don't mean to stomp on anyone's toes. Some of the paragraphs seemed to confuse manifold vacuum (inherent in the use of throttles) with venturi vacuum (inherent in the use of carburetors), so I corrected that too. Cheers, zowie 18:34, 25 October 2005 (UTC)

It's not true that the engine in the Toyota Prius has no throttle. Certainly it should throttle less, due to the valve timings and low-power engine, but the throttle is still there. --KJBracey 23:39, 23 November 2005 (UTC)

Re-edited out where someone modified an article to say that modern atkinson engines run without a throttle. If its a gas engine and in production is has a throttle. Most test-bed gassers have throttles as well excluding GDI test engines, but that has absolutely nothing to do with variable valve timing or atkinson.Theunchosen (talk) 12:26, 18 May 2009 (UTC)theunchosen

Doesn't manifold vacuum reduce the effective compression ratio?[edit]

For example, if you have an engine with 10:1 compression with ½ bar of manifold vacuum, isn't the effective compression ratio only 5:1?

If so, could a flexible fuel engine be given a nominal compression ratio appropriate for the highest octane fuel (e.g. ethanol), avoiding detonation with other fuels by limiting throttle opening and thus preventing effective compression from reaching a value that supports detonation? 76.80.9.100 22:20, 21 March 2007 (UTC)


That's essentially true, but you're using old-school thinking, which makes it more difficult to understand.

Think in terms of (absolute) manifold & cylinder pressures, air density, and mass air & fuel flowrates.

(Geometric) compression ratio confounds understanding because of the widely-variable intake conditions, which vary with not just throttle position but also ambient temperature, barometric pressure, elevation (or altitude, in aircraft engines) & humidity.

It's not compression ratio that causes pre-ignition, but a combination of cylinder pressure, cylinder temperature, (which increases with increasing compression ratio) air:fuel ratio, spark timing and initiators such as red-hot spark-plug tips or red-hot carbon particles. Preventing detonation can be achieved by adjusting any one of these variables.

Modern (c.1996-2010 or so) gasoline/flex-fuel engines use a fairly-high geometric compression ratio, in the neighborhood of 12:1, and prevent detonation by retarding spark timing and/or with variable valve timing.

Direct injection makes pre-ignition an obsolete and irrelevant concept; ignition cannot occur until after there's fuel in the cylinder. HCCI engines rely on auto-ignition and can achieve stable combustion with air:fuel mixtures that are significantly leaner -- often double -- than stoichiometric

This is not the same as Diesel engine ignition. Diesel fuel has a significantly lower autoignition temperature than gasoline or ethanol and Diesel engines have a significantly higher cylinder temperature due to compression. 198.208.251.24 (talk) 19:02, 30 May 2014 (UTC)

Compression braking[edit]

"More vacuum is created in some situations. On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast because the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as compression braking, is often used in engine braking to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). Note that although "compression braking" and "engine braking" are sometimes used to describe the same thing, "compression braking" here refers to the phenomenon itself while "engine braking" refers to the driver's usage of the phenomenon. Compression braking can be greatly increased by opening the cylinders to the exhaust with a valve on downstroke while in overrun, which is often done on large trucks (see Jake brake)."

No. It is called compression braking for a reason. What is described here is braking due to vacuum (actually much by engine friction). This is how engine braking works in most cases. Unless the engine has a "Jake brake", it cannot brake using compression...any energy absorbed compressing the charge is returned to the crank immediately as the compressed air functions as a spring and forces the piston back down again. Compression braking requires a device that opens the exhaust valve at TDC, allowing the compressed air to escape before it returns its energy to the crankshaft, i.e. a Jake brake (read it!). These are used on diesels because they cannot brake by vacuum, lacking a throttle. The 3rd choice is an exhaust brake that closes the exhaust so the engine has to force the exhaust out...the opposite of vacuum braking. Vacuum braking uses intake stroke. Compression braking uses compression stroke. Exhaust braking uses exhaust stroke. Three unique ways to achieve engine braking. .45Colt 04:08, 15 April 2015 (UTC)