Exhaust Brake

06Fargo wrote:
Why don't gasohol engines in pickups have exhaust brakes?

1st road trip with work F1 fitty ECO boost I thought truck go crazy! the trans shifted down to a 4000 rpm engine gear to hold downhill speed on cruise...

Even though both gasoline engines and diesel engines are piston/crank/compression/combustion engines, they're really nothing like each other and use completely different principles to function.

So without getting too technical..... Most gasoline engines use vacuum to fill the cylinder under atmospheric pressure. To achieve that vacuum gas uses an intake venturi throttle flap to regulate air flow at a given time. When the throttle is closed vacuum builds behind the flap and causes the engine to work against this vacuum, thus you feel deceleration. The higher the rpm's with the throttle closed, the higher the vacuum against the piston trying to suck in air flow. But since vacuum is only so strong the resistance is only so great. Before anyone jumps to conclusions too, even forced induction gas engines use this same principle system.

All that said, if you installed an exhaust brake on a gasoline engine you would find that the intake and the exhaust restriction would conflict each other and would result in very little gained in additional retarding power due to the fact that the intake flap is restricting most of the air and the exhaust flap is trying to restrict more air than what is barely being sucked in. So in order for the system to work at all you'd have to allow the intake some amount of air flow just to have the exhaust have something to pressurize. And since gasoline engines are very sensitive to air/fuel ratio, the system would have to incorporate some sort of an anti-fueling mode during that time when the throttle is partially opened a bit so you didnt run into a hyper-lean condition. You can see where this would be getting really involved and costly for the consumer. And really, to end most discussions about exhaust brakes and engine brakes mounted on gasoline engines, the bottom end structure and component system of a gasoline engine isnt strong enough to support the pressures and stresses it would face and will eventually result in failure.

Hope I didnt go off in a tangent and get too confusing.....