OT: Pinking Diesel Engine

[Johnny B Good]

Yes. Effectively, the 'accelerator' pedal is controlling the speed governor limit in the fuel injection pump on Old Skool diesels. This is why, with a modicum of care, you can gently ease in the clutch at tickover to start the vehicle moving on a level or slightly uphill inclined stretch of road. Also, you don't have to 'work' the accelerator pedal as much as in a petrol engined vehicle to maintain a steady cruising speed up hill and down dale. However, this no substitute for cruise control where you wish to cruise right on the speed limit.

A good comparison between petrol and diesel engine responses to the accelerator pedal can be made using a DC motor as the analogy whereby the petrol engine's behaviour is more like using a rheostat to control current flow to a PM DC motor versus using a voltage converter that applies a steady voltage determined by the user's voltage control setting.

In the former, it takes very little current on no load to make the motor run at 70% or so of the voltage supply's limiting speed (the voltage applied when you reduce the series resistance of the rheostat to zero ohms). Applying a load under this condition will dramatically reduce the motor speed (perhaps even stall it) unless you compensate by allowing more current to flow by reducing the resistance in the rheostat. In this respect, controlling the speed of a petrol engine is on a par with trying to control the speed of a PM DC motor under varying loads using a variable series resistor.

In the case of a (n Old Skool) diesel engine, the behaviour is analogous to using a variable voltage stabiliser to maintain a constant voltage at the PM DC motor's terminals. Unless current compensation is used[1], the speed will sag slightly with increasing load due to volt drops internal to the motor's armature and brush gear resistance. The sag in speed is typically a matter of a few percent versus the rheostat controlled case where a 50% or greater loss in speed can occur.

[1] It is possible to give the voltage stabiliser a negative impedance characteristic to cancel out the motor's impedance so as to maintain speed over its normal loading range. However, this negative impedance needs to be less than the motor's impedance to avoid runaway overspeeding (the maximum voltage will provide a speed limiting effect anyway in this case - you'll simply have lost stable speed control, changes of controller input will cause wild and uncontrolled voltage swings)

[NY]

All diesels that I have driven, both old skool ones like an early 1990s VW Golf, which was very agricultural and newer ones such as Peugeot XUD and 2.0 or 1.6 HDi and Honda Civic 1.7 and CRV 1.6, have been dead easy to set off up any hill with your foot off the throttle by letting the clutch in. That's what makes them so easy to drive. I've even done it on a 1:3 hill (Chimney Bank, coming out of the village of Rosedale Abbey in North Yorkshire) when the car in front of me stalled and I had to do a hill start on a steep bit. Foot off throttle, clutch up to bite to hold car, handbrake off, clutch up a bit more and starts to crawl forward; if I'd had to use more throttle I'd have had to slip the clutch more to keep the car going slow enough not to hit the lethargic car in front. After I worked out that he was going to keep stalling, I hung back and kept a sensible gap from him so I could go at idling speed in first (as opposed to having to go even slower than that) and kept stopping as I caught up with him. I'd love to know how that car managed to go *so* slowly, given that my car in first at about 800 rpm was going faster than him. Was he revving the engine enough to stop it stalling but then slipping the clutch to achieve a speed of probably less than 2 mph. There *was* a bad smell of hot clutch coming from his car.

The single exception to the ability of starting with no throttle is the VW Golf Mark 5 PD engine, which seems to have some weird logic built into the ECU which forcibly stalls the engine by cutting off the fuel (that's what it feels like) if you don't have enough throttle, whereas all other diesels will make a valiant effort to kept going and give you chance to apply more throttle if you sense that the engine is struggling. Not so on the Pumpe Duse engine which stalls with no advance warning - even more easy to do than if you temporarily drive a petrol engined car when you're used to driving a diesel. I was all set to buy a Golf Mark 5, because the engine is nice in all other conditions and the car is nice, until I encountered the infamous PD-stall. It's the only car of many petrol and diesel that I've driven over the years that I have stalled repeatedly in traffic.

[Tim+]

I suspect that "sudden death" feature built into some modern diesels is to protect the DMF from the effects of very low revs.

Tim

[Cursitor Doom]

Same thing happens with modern common-rail diesel engines; there's absolutely no difference.