Can anybody tell me if (Turbo)Diesel engines are like petrol engines and require some kind of 'choke' on cold start up ? Or do they just fire up and thats it - idle curiosity on my part.
Also I am surprised by the lack of engine braking on the motorway as I keep bowling up behind things after having lifted off expecting the engine to halt my progress more than it does. I thought the higher compression ratios would give me more engine braking than say my 1.8t Golf that I used to drive.
Puzzled P !
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No choke on diesels, turbo or not. Just turn the key and go.
Can't answer the second question as it is so long since I have driven a petrol engine.
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Don't forget to wait for the glowplug light to go out if your engine isn't direct injection.
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there is little in the way of engine braking at tickover a diesel could still pull a caravan up a hill
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>>at tickover a diesel could still pull a caravan up a hill
Indeed, only because, under the application of a load torque, the governor increases the fuelling to maintain tickover speed. Without any correction, tickover on any engine is where the engine produces no useful torque - the fuel burnt just overcomes the losses. If there were extra torque available, the engine would speed up - tickover or engine speed in general is simply equilibrium, in a rotary sense.
On older diesel engines in cars, there was a damper fitted to restrict the rate that the throttle closed, because there was too much engine braking.
I think the lack of engine braking may be influenced by gear ratio, as Phil suggested, by the engine management, and by the possible incorporation of a throttle valve.
number_cruncher
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It is indeed the result of high gear ratios and electronic management at work. The Goona will cruise down the road quite happily in 6th at 33 mph with feet off. Dip the clutch and you can see what the engine management was trying to do as the revs rise to just under 2k before dropping back to idle.
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Can't say I have noticed lack of engine braking but I wonder if it is because most diesel engines nowadays are so high geared in relation to their revs in top gear. I always thought that engine braking in diesels was much better in lower gears, and less likely to "run away" compared to petrol - but I will stand corrected by those who know rather than guess (like me!)
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"...No choke on diesels, turbo or not..."
There should be a thermostatically controlled fast-idle sensor and cable on the fuel injection pump, which serves much the same purpose as a choke on a petrol engine. All automatic, but if faulty you would experience cold-running problems just like having a faulty choke.
Ian
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Not on my high pressure common rail there isnt.
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>Not on my high pressure common rail there isnt.
Yes, there is, but it will be controlled by the ECU. Probably gives different timing to help keep the clatter down, and alters the amount of fuel injected to keep the idle speed up.
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RichardW
Is it illogical? It must be Citroen....
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No there isnt. There is no "thermostatically controlled fast-idle sensor and cable on the fuel injection pump, which serves much the same purpose as a choke on a petrol engine"
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>>Not on my high pressure common rail there isnt.
>>Yes, there is, but it will be controlled by the ECU. Probably gives different timing to help keep the clatter down, and alters the amount of fuel injected to keep the idle speed up.
No there isnt. There is no "thermostatically controlled fast-idle sensor and cable on the fuel injection pump, which serves much the same purpose as a choke on a petrol engine"
Hi, RW and RF I reckon you are both right, common rail systems adjust the timing and volume-per-squirt of the injectors plus if necessary add an extra squirt or two per compresion stroke based on all sorts of information that the ECU gathers and processes, this of course includes coolant temperature and inlet temperature as well as throttle position, air mass, turbo boost etc etc etc.
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Oh yes - I have one of those!
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Touche!
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RichardW
Is it illogical? It must be Citroen....
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Thanks Guys - interesting stuff. I think it must down to the very long gear ratios as you say.
I still cant really get over the fact I am driving a Diesel, only 4 years ago I would have just laughed at anyone who suggested it. But the reality is that my new car wipes the floor with my old one in every respect IE performance and economy and I actually find it more relaxing to drive.
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cheddar - its a BMW 120d
....runs for cover, puts on flak jacket etc - someone tell me when its safe to come out !!
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BMW 120d should be pretty quick!, reckon the engine braking matter is to do with gear ratios as much as anything, at 70mph you are pulling just over 2000 rpm in the BMW where as a Golf 1.8t would be around 3000 at the same speed.
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I checked today 1800 = 70 mph / 2000 = 75mph
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I checked today 1800 = 70 mph / 2000 = 75mph
Hence little engine braking!
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I thought the higher compression ratios would give memore engine braking than say my 1.8t Golf that I used to drive.
I was surprised by this too when I first started driving diesels, for exactly the reason that you mention. You'd think they should almost stop dead?
We have advanced driver training at work and one of the instructors said he's noticed that too, but the effect seemed to vary a lot from car to car (of the same type).
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