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Christ · 05-23-2010, 12:34 AM

Where are you testing vacuum? This is very important. If you're testing vacuum at the manifold, vacuum will drop when EGR is introduced, because EGR is flowed by vacuum.

EGR does not add volume to the intake. If it did, that technically would increase the volumetric efficiency of the engine. Ok, not really, because for that to happen, EGR would have to be combustible. However, in theory, it would increase VE because, assuming the engine was already running at peak VE, filling the cylinder 100% on every intake stroke, then introduction of EGR would have to be under pressure (which it is not, at least not significantly) and would then cause more than 100% of the available capacity of the cylinder to be filled. This, however, is not the case.

EGR is designed to characteristically limit the mixture available to a cylinder by introducing a percentage of the cylinder's volume as inert gas, which then only allows (given a specific VE for that stroke) the cylinder to be filled less with overt fuel/air mixture.

Because of this, throttled control of a gasoline engine can be kept with only EGR flow. No throttle plate is actually necessary.

When you say that a vacuum leak in the manifold allows more air into the cylinder, I presume you're respondent to the fact that a vacuum leak will cause a lean condition. It does not, however, improve the VE of the cylinder being tested. The cylinder can still only draw in X amount of mixture per intake stroke, and because you now have a vacuum leak, more air will be drawn than fuel (per ratio), so the resultant exhaust gasses will show a lean condition for that cylinder.

A test you can do to confirm that vacuum drops with introduction of any gas is to measure the vacuum force at the end of the hose on a shop vac which has the variable vacuum control sleeve installed. While measuring the vacuum drawn at the end, begin to slowly open the sleeve, and observe the vacuum at the "throttle" area drop. Measure output at the exhaust port of the shop vac, and it remains the same, even though measured vacuum at the intake port has decreased.

05-23-2010, 12:34 AM	#24 (permalink)
Christ Moderate your Moderation. Join Date: Nov 2008 Location: Troy, Pa. Posts: 8,919 Pasta - '96 Volkswagen Passat TDi 90 day: 45.22 mpg (US) Thanks: 1,369 Thanked 430 Times in 353 Posts	Where are you testing vacuum? This is very important. If you're testing vacuum at the manifold, vacuum will drop when EGR is introduced, because EGR is flowed by vacuum. EGR does not add volume to the intake. If it did, that technically would increase the volumetric efficiency of the engine. Ok, not really, because for that to happen, EGR would have to be combustible. However, in theory, it would increase VE because, assuming the engine was already running at peak VE, filling the cylinder 100% on every intake stroke, then introduction of EGR would have to be under pressure (which it is not, at least not significantly) and would then cause more than 100% of the available capacity of the cylinder to be filled. This, however, is not the case. EGR is designed to characteristically limit the mixture available to a cylinder by introducing a percentage of the cylinder's volume as inert gas, which then only allows (given a specific VE for that stroke) the cylinder to be filled less with overt fuel/air mixture. Because of this, throttled control of a gasoline engine can be kept with only EGR flow. No throttle plate is actually necessary. When you say that a vacuum leak in the manifold allows more air into the cylinder, I presume you're respondent to the fact that a vacuum leak will cause a lean condition. It does not, however, improve the VE of the cylinder being tested. The cylinder can still only draw in X amount of mixture per intake stroke, and because you now have a vacuum leak, more air will be drawn than fuel (per ratio), so the resultant exhaust gasses will show a lean condition for that cylinder. A test you can do to confirm that vacuum drops with introduction of any gas is to measure the vacuum force at the end of the hose on a shop vac which has the variable vacuum control sleeve installed. While measuring the vacuum drawn at the end, begin to slowly open the sleeve, and observe the vacuum at the "throttle" area drop. Measure output at the exhaust port of the shop vac, and it remains the same, even though measured vacuum at the intake port has decreased. __________________ "¿ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"