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A pal of mine has a 52 reg manual JTD Alfa and he tells me that his car has no engine braking. His dad has a Fiat with a JTD engine and when he bought it he apparantly commented about the lack of engine braking. We were talking about this and I suggested that these cars might have a freewheel facility. My pal said they didn't and it was down to fuelling. I didn't understand this and said I believed that engine braking resulted from resistance to turning due to good compression especially as they are diesels. Any comments would be appreciated as I would like to understand this phenomenon. Best Regards.
Edited by Dynamic Dave on 05/02/2009 at 18:34
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If the engine had no losses, the same pressure you get from compression would drive the piston down on what would have been the power stroke, and despite the engine having compression, there would be *no* nett engine braking from that source!
Engine braking represents losses - these losses would include driving the oil pump, the water pump, alternator, power steering, air conditioning, and internal friction losses, viscous losses, aerodynamic losses, and the heat lost by the compressed air during the compression stroke to the coolant.
On a petrol these would also include the losses incurred by the pressure drop across the throttle valve.
In this respect engine braking represents the vehicle's kinetic energy becoming low grade heat energy in exactly the same way as in the vehicle's brakes - the *only* difference is in the wear of the friction material which does not happen in engine braking.
Do these engines have variable valve timing?
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I asked that but my pal seemed not to know. I also stated that I couldn't see the point of lowering compression on the overrun and making things more complicated than they need to be. Regards,
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It's quite common on petrol engines to reduce the throttling losses on over-run. On overrun, the fuel is cut off, and the idle speed control valve (or drive by wire throttle) is opened to allow the free flow of inlet air.
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But this and his dad's are diesels. Thats what I find hard to understand. Regards.
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>>I couldn't see the point of lowering compression on the overrun and making things more complicated than they need to be.
If the hardware and the controller are already there, it's not so much more to add extra over-run logic in the software.
Why do it?
Economy and smoothness.
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I agree. I don't know why or how it is possible to set valve timing/opening to make for smoothness but I would want engine braking for general driving anyway. That makes for smooth deccelaration when it is needed especially downhill. Regards.
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slightly off topic but related i think, my daughters driving instructor (more importantly the examiners) teach block changing up and down the gearbox and dont let the gearbox take the strain, far cheaper to replace a set of pads than clutch/gearbox and saves fuel this is the current thinking from the goverment/ministry so i wonder if the vehicle makers are doing the same visa ve no engine braking this would also increase the advertised mpg making more saleable
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>>I would want engine braking for general driving anyway.
There, we differ - I enjoy driving without engine braking, and being in control of the level of braking myself, rather than using the essentially fixed amount of braking given by the engine.
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