Gas engines in school buses coming back?

Groover wrote:

dodge guy wrote:

RoyJ wrote:

msmith1.wa wrote:
Besides the Ford V10, the other player in the gas bus motors is PSI. They have an 8.8L (540 ci) that can be used with gas, or propane. I am not sure if it can be set up for CNG. This motor is basicly an improved and stroked chey 8.1L and is likely the larger than 8L engine that GM has been teasing. Navistar makes bus chassis's with this motor.

Glad you mentioned this, I posted this in another thread, showing just how usable the power curve of the PSI 8.8 is:

PSI 8.8L
Rated HP: 270HP @ 2600
@1000 RPM: 103HP
@2000 RPM: 215HP

Rated Torque: 565 ft-lb @ 1500
@1000 RPM: 540 ft-lb
@2000 RPM: 564 ft-lb

That's very diesel like. And should have decent service life at such low rpm.

And decent mileage with such low rpm!

There is a catch to large displacement, low RPM motors. Every gas engine that I have personally checked idles around 600RPM and 19 inches of vacuum. This basically means that fuel consumption at idle is proportional to displacement so the big motors are fuel hogs at low speeds. Maybe PSI has found a way around this by lowering the idle speed and shifting the entire power curve down or cylinder de-activation at low speeds. I don't know but it would be interesting to find out. Also, a low speed motor may need more gears in the transmission and gear shifts.

19” of vacuum is about 1/3 of an atmosphere. So a 8.8 liter engine idling is only drawing about 3 liters of air every 2 revolutions. I wouldn’t expect these engines to consume much more fuel at idle than the old carbureted big blocks. They will, of course, take more fuel to idle than a diesel but only because diesels can operate at a much higher air/fuel ratio than gas engines.