my view on engine thermal efficiency

[dkjones96]

I've recently made two trips from Albuquerque to LA. One in my car and another in a 2006 Grand Marquis for business. My 2.0 tracker got an average 24.5mpg with an average speed of about 80. The GM got average 28.3 at 85. These were an out there and back, leaving Saturday and back Sunday, trips taken one week apart(1745 miles round trip).

My car turns 3950 rpm to keep 80, and for numbers sake, don't count the fact that around Flagstaff, AZ at 7000ft I went a total of almost 50 miles stuck out of overdrive because the little engine couldn't keep that speed without being pretty much wide open in 3rd(5200 rpm at 80) for that stretch. That calculates out to 355,588 pulses per mile(true for any speed in overdrive with a locked-up torque converter.

The grand marquis turned 1900 at 85 and didn't need to get out of that in AZ but a couple times. That car runs 321,806 pulses per mile. Double the cylinders and a little more displacement and weighs about 1000 more than the tracker does. It's a boat.

The Tracker not only uses more pulses per mile but the pulses it does use are harder. Granted, the GM had an unfair advantage in this one because it makes peak torque very low and is close to where it was running the whole time. The tracker makes peak torque at 3000 rpm and the engine was revved out past that because of gearing to start with.

Nevertheless, just making an engine smaller and revving higher and using stronger pulses doesn't always work. The GM in this example was hardly ever over 20% throttle. The other car was almost always above 50%. The tracker used 9.5% more pulses but got 13% worse mileage and that's comparing average of 80 vs 85. And as you slow down the span stays about the same or gets worse. At around 60 the GM was reading about 32-34mpg and the tracker never read over 26mpg and anything over 55.