Compression Ignition Addict
Join Date: Dec 2010
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My 03 Duramax without an exhaust brake did very little holding back. I put it in low gear to descend a 20 mile 8% grade in BC. I was pulling a 15000 lb trailer. The truck accelerated until the engine reached nearly 5000 rpm. Two things happen as the truck and engine speed up that work together to prevent further acceleration -
1.) as the engine accelerates it pumps more air through the exhaust system which produces some back pressure even without an exhaust brake - this back pressure tries to prevent the piston from rising up on the exhaust stroke creating a negative torque on the crankshaft. This torque is then multiplied through the gear ratios of the transmission and differential and if it is equal to the torque created by the weight of the truck and trailer on the incline the truck will stop accelerating.
2.) as the speed of the truck increases more air resistance is produced so there is less torque required to prevent further acceleration.
The amount of rear axle torque needed to prevent the truck from accelerating can be easily calculated if the weight of the truck and trailer are known as well as percent slope of the grade.
The amount of torque an exhaust brake will produce can also be easily calculated once you know the engine displacement and the back pressure in the exhaust system. Because none of the exhaust brakes allow high back pressure to be produced, in order to prevent engine damage, there cannot be allot of negative torque produced at the engine crankshaft by this type of engine brake. The thing is this low torque can be multiplied through the transmission by gearing down. Because gearing down both increases the back pressure (and therefore increases crankshaft torque) and multiplies this torque through the transmission it is essential that gearing down be involved in engine braking with this type of engine brake. I suspect the new Ford will do just fine in holding back a load once we get it in our heads that we need to manually gear down with heavy loads.
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Last edited by 4x4ord; 04-04-2011 at 12:05 PM.