KING@KESTREL (04/19/83)
From: Richard M. King <KING at KESTREL> I've made some intersting observations about the EPA highway ratings. They are notoriously inaccurate. However, I found out a few things. 1> they are much better for Diesel cars. 2> the people who do the testing DO take air resistance into account. Although the tests are run on a dynomometer, the setting on the dynomometer is made to simulate actual measured air resistance. 3> rolling resistance is, of course, taken into account. The car does actually roll on the dynomometer. The surface is actually roughened to simulate road conditions. So why are the test results inaccurate? And why are they worse for gasoline engines than Diesels. I have a hypothesis... It seems to me that the main difference between test and reality is that the air is not rushing at the front of the car at 55 MPH during the test. This does NOT cause extra air resistance; the wind tunnel test that produces the drag setting for the dyno would measure the air resistance that the front of the car encounters. (It is not reasonable to assume that wind tunnel tests are done with some sort of dummy engine. If that were the case the tests would be as inaccurate for Diesels as for gasoline cars.) I hypothesize that the air turbulence under the hood reduces the efficiency of gasoline cars, but not of diesels. (Why one but not the other? Variations in air pressure going into cylinders shouldn't affect diesels as long as there is enough air to burn the fuel, which would always be true except near full "throttle". Gasoline engines, however, are very sensitive to mixture and therefore to air pressure. Servos that modern engines contain to regulate fuel feed would be much too slow to follow the variations I am hypothesizing. I expect 10 Hertz variations.) Engine designs tend to be tested in a non-windy room. Does anyone out there know anything about this possibility? Dick -------