quote:

Corey, re: engine 'braking'.
If 17.0:1 is not high compression, what is?
http://www.cumminsdieselspecs.com/12v.html

Can any over-the-road gasoline powered vehicle engines maintain compression that high?

Mogan,

It's more a case of using the engine developed compression. A gas motor always has compression braking when the air intake is restricted, but a diesel engine doesn't have that feature. An artificial means of restricting air flow is therefore required.

An exhaust brake or a Jake Brake (valve manipulation) is required to use the piston developed compression.

Several gas electric hybrids are using four stroke Atkinson cycle engines with some fairly high comp ratios. The Prius engine gets up to 13.0 to 1 at its' maximum. This is admittedly hardly a typical street application. Don't see hardly any above 12.

http://en.wikipedia.org/wiki/C...release_engine_brake

In a sense, the 'Jake' brake RELIEVES compression, whereas a gas engine's throttle body strangles the airflow.

Yes, now you have it.

quote:

Originally posted by Steve VW:
Several gas electric hybrids are using four stroke Atkinson cycle engines with some fairly high comp ratios. The Prius engine gets up to 13.0 to 1 at its' maximum. This is admittedly hardly a typical street application. Don't see hardly any above 12.

The automotive engines that claim Atkinson cycle are not true Atkinson engines. The true Atkinson engine, varies the stroke on intake and compression cycle and then lengthens the stroke for the power cycle.

There is no variable stroke Atkinson engine in any hybrid engine on the road. The hybrid engines all use valve timing to vary the effective intake/compression cycle.

We can expect to see an increase in gas engine compression ratios in the future as it will be easier to do with direct fuel injection, similar to diesels. Detonation is currently the limiting factor with any sort of fuel induction outside of direct injection.

The main reason the diesel engine does not provide effective engine braking is that it will always gulp in a full charge of air to compress (no throttle plate) With no fuel added, that same charge of air, now compressed, will help force the piston down during it's normal power stroke cycle. Even though some energy is used to compress that air, most of that energy is returned back to the crank. By modifying the valve timing as in the Jake brake system, the compression cycle is normal and a full gulp of air is forced into the engine and then the exhaust valves are opened as close to peak compression after TDC to release the force thru the exhaust and taking that built up force away from the crank. This converts the engine to a large air pump during application.

Exhaust braking works by blocking the output of air during the exhaust cycle. Yes, in this system while air is being forced in during the intake cycle and energy is returned to the crank during the power stoke, when the exhaust valve is opened This charge of air now is forced out against a block thereby taking the stored energy out of the cycle.

The question about whether all diesel powered Barths had air braking systems, IIRC some of the shorter GM diesel V8 powered Barths had hydro-boost systems. Rusty?

I have seen some coaches (SOB) with full air but had disc brakes on all four corners. Not a real popular system.

So, just for grins, let's look at the '90 wide body on ebay. It has a 3208 Cat and air suspension. Gillig chassis. Can we assume this has air brakes? Normally, I'd ask the owner, but son seems to be unfamiliar with the coach.

Just wondering....

Yes, I would assume full air on that one. A big clue are the 22.5 wheels, if it's got those it's full air.

Thanks.

Be careful not to take Corey's statement any farther than possibly a larger Barth with a Gillig chassis and air suspension. Having 22.5" inch wheels does NOT guarantee air brakes. I have that size wheel on my 33' Breakaway. It has Air over Hyd. brakes.

Thanks Bruce, myth busted! I wouldn't have thought the Breakaways got 22.5 without full air.

The Barth Breakaway has disk brakes at all four corners. In the time period these were built I am not sure that there were full air disk brakes available on anything. All the full air brake systems used drum brakes, for the most part they still do today.

A side note on high compression. Minneapolis-Moline farm tractors with the propane engine had compression ratios over 14:1. They would do more work on less fuel than either the gasoline or diesel versions.

The octane rating of propane is about 135 so no pre-ignition. This is also the reason you can add propane to the air supply of your diesel engine as it will not pre-ignite.