OT: Turbo diesel engines

[bcrazy]

This may be the wrong forum but the activity seemed low in the OT forum so i'll try here

In these diesel engines times i wonder why so few manufacturers make a diesel engine that i actually could like.... :)

Most modern diesel cars i've driven have a ketchup like torque curve that starts from just below 2k and lasts to about 3-3.5 k. This is all nice while it lasts, but below 1.5-2k absolutely NOTHING is happening and these engines (in general) stall very easily from a standing start, unless you keep revs above say 1300 rpm's,if you go below this it just shuts down without much warning.

You can forget rolling away in 2nd gear from 1000 rpm's like i do in my NA petrol 940. I can easily perform a standing start in steep uphill BELOW 1000 rpm's in my old volvo. I know this is not normal on modern petrol engines but still it is very easy to get used to having so much off idle torque (or is it just the montstrous heavy flywheel?)

But there are some exeptions, like the Ford transit connect with a 1.8tdci engine and 75 (only)bhp. This is low on power but pulls like a stomper just off idle.

While reading some specs on TD engines i noticed the following.


1. High hp turbo diesels have low comp. ratio from 17-18.5:1

2. Low hp turbo diesels have high comp. ratios from 18.5-20:1


So the BIG question is:

Why isn't there a high hp turbo diesel engine with like 20:1 comp ratio and gobs of off idle torque and plenty hp aswell???

Afterall detonation is impossible so why lower comp. ratio when using high boost pressures?

[JW240]

afaik the lower the CR to get the engine to run smoother, to prevent the diesel "hammer" at the beginning of the combustion cycle. With CDI systems you can give some pre-injection to prevent that. From what i've read detonation is possible. well, preignition is possible. whats the difference.
The second generation D5 even has a throttle valve... the start to look like petrol engines lol.

have you driven D5's? (kinda a duh question maybe) we have (had) both the old and the new D5 but both with automatic gearbox so i cant tell how it performs at anything sub 1.5K rpm. It runs great from 2-4.5+K though. The new generation has less turbo lag, runs smoother, a lot faster (stock about as fast as a chipped old one if you get the 185 hp version). They lowered the CR though.. fuel economy isnt nearly as good with this newer D5 even with the new 6 speed gearbox...

[242Bleek]

You are in Oslo? I guess you haven't seen what some of the american full size turbo diesel trucks are all about yet. Think, Cummins Powerstroke, Duramax. They are making crazy HP and torque even stock with lightning fast spool, so much that they are phasing big gas V8's out of big trucks all together

[barefoot]

In these diesel engines times i wonder why so few manufacturers make a diesel engine that i actually could like.... :)

Most modern diesel cars i've driven have a ketchup like torque curve that starts from just below 2k and lasts to about 3-3.5 k. This is all nice while it lasts, but below 1.5-2k absolutely NOTHING is happening and these engines (in general) stall very easily from a standing start, unless you keep revs above say 1300 rpm's,if you go below this it just shuts down without much warning.

:shrug:

My Peugeot is pretty flat below 1500 RPM, but it chugs away and will get you moving eventually. It goes "whoosh" at about 1800RPM, and runs constant torque up to 4000RPM. It keeps hauling up to redline...can't remember whether that's 5500 or 6000RPM.

Not many petrol engines will pull from 1300RPM either, and they're generally flat and lifeless until you hit 3500...at least.

For general driving, I like having all the torque I could possibly use, right in the guts of the rev range where I would be anyway. Drive normally in the 2000-3000 range, and always be just a pedal squeeze away from being sucked back in my seat.

tim
zzeeiiiwhoooooosh!

[canadiangt]

I own a 2002 VW TDI. I will admit that I have big nozzles and a self made "Tuning box", but it hauls some serious a$$. It will start in 1st gear with no throttle, but 1st gear is so low that it is almost useless. Once you get into 2,3,4 the torque is unreal and it pulls and pulls. The torque curve for thes engines is really flat and very driveable. The VNT (Variable Nozzle Turbine) turbo make it spool very quick and there is 10 psi plus available from <1500RPM. I think that the key for the low rpm torque is the VNT.
I would have to say that my only real complaint is that the 1st gear is so short that it really makes you aware of the 4300 RPM torque dropoff. Modified at about 120hp, these engines can put out over 200 lb-ft of torque.

[JohnMc]

I'm not sure how preignition can occur, there jsut isn't any fuel to burn before the injectors start spraying it in, and the injector timing is done with the idea being that the fuel will begin combusting as soon as it is sprayed.

I thought the idea about lowering the CR on 'high perf' diesel was so they could run higher boost levels, and trying to keep the peak cylinder pressures under the level that starts bouncing the head up off the block and blowing the HG.

[MikeHardy]

the pugs at work (307, 807 both HDI) can accelerate normal up through gears to 30mph in 5th idling without touching the throttle

[1stRaven]

I know that when the throttle cable snapped in my 740 with the td engine, I got it home at 1000rpm only. At one point, I was in 4th, doing 30 miles an hour.

and that engine had plently of torque low down. Use to be able to do a hill start with no throttle input at all.

[MikeHardy]

i think most of it is down to really strong/good idle speed control/electronics, the VW caravelle is much stronger then pugs at work just off idle but ECU cuts out much sooner when you load it up at idle making it easier to stall.

[JohnMc]

I'm not sure about other diesel engines, especially the newer electronically driven ones, but the diesel motor in my Dad's 65-ish Land Rover was RPM regulated by the accelerator pedal, not power regulated. The position dictated a certain RPM, and the injector pump would add more fuel if the motor was turning slower, reduce fuel if turning faster. So if you put a hafty load on the motor at 'idle', it could conceivably go to full power output trying to keep the motor turning at 800 rpm (or whatever idle was set at).

This worked well in a couple of scenarios. First, off road, where the motor would automatically power up and down as you cralwed up and over trail obstacles. Just leave that pedal in one position, and the motor did all the rest. Secondly, on the road, there was a lever on the dash that you could set to hold the pedal down a certain amount, and it worked perfectly as a cruise control. Up and down hills, it would maintain the same speed. Well, granted, LWB pickup Lanr Rover, 2.25 n/a diesel motor, and steep rural Missouri hills, it would slow down sometimes...

[Hank Scorpio]

You are in Oslo? I guess you haven't seen what some of the american full size turbo diesel trucks are all about yet. Think, Cummins Powerstroke, Duramax. They are making crazy HP and torque even stock with lightning fast spool, so much that they are phasing big gas V8's out of big trucks all together

Hi this is planet Earth calling, do accept the charges?

- First off, a 5.9+L motor is a bit tougher to compare against any of the diesel 4/5bangers found in passenger cars.

- Secondly, haven driven 5 different years of Cummins Dodges, none of them have had "instant" spool. They are all a bit doggy right off idle, but having 400+ cubic inches of displacement helps.

[canadiangt]

The electronic control on the new diesels is pretty unreal. The VWs actually measure the crank acceleration after each injection event so that they can balance the cylinders for a nice idle. The accelerators on most, if not all of the current diesels is throttle by wire. The throttle pedal is actually called the torque request input, so every throttle position actually corresponds to a load condition.
I hope that the US soon complies to ULS diesel standards so we can start getting in cool euro diesels. There is an Audi A4 TDI.

[bcrazy]

:shrug:

My Peugeot is pretty flat below 1500 RPM, but it chugs away and will get you moving eventually. It goes "whoosh" at about 1800RPM, and runs constant torque up to 4000RPM. It keeps hauling up to redline...can't remember whether that's 5500 or 6000RPM.

Not many petrol engines will pull from 1300RPM either, and they're generally flat and lifeless until you hit 3500...at least.

For general driving, I like having all the torque I could possibly use, right in the guts of the rev range where I would be anyway. Drive normally in the 2000-3000 range, and always be just a pedal squeeze away from being sucked back in my seat.

tim
zzeeiiiwhoooooosh!

What kind of engine do you have?

The pug diesel engines i know off all starts dropping torque past 3K.

With the monstrous torque that most modern diesels make at low rpm if this where to stay flat to 4k most of them would be making close to 200 hp at 4 k and not 120-140 for an average 2.0 L turbo diesel.

[745 TurboGreasel]

While reading some specs on TD engines i noticed the following.


1. High hp turbo diesels have low comp. ratio from 17-18.5:1

2. Low hp turbo diesels have high comp. ratios from 18.5-20:1


So the BIG question is:

Why isn't there a high hp turbo diesel engine with like 20:1 comp ratio and gobs of off idle torque and plenty hp aswell???


Because you need to buy a chip and injectors to tell the car you desire exhaust smoke smoke and drivetain breakage?

[bcrazy]

Because you need to buy a chip and injectors to tell the car you desire exhaust smoke smoke and drivetain breakage?

Why would a flatter torque curve wich would mean more hp break the drivetrain?

You still have the same amount of torque but only for longer.

I thought that high torque at low rpm was harder on the drivetrain than the same torque at high rpm?

[Janspeed]

BMW 335xd touring... awesome!
http://faimg1.forum-auto.com/mesimages/228078/sans%20titre5.jpg

[barefoot]

What kind of engine do you have?

The pug diesel engines i know off all starts dropping torque past 3K.


I have the '06 revision of the 2L common rail turbodiesel.

Main improvements over the earlier engines was going to 16 valves and the variable geometry turbo.

Those two changes alone will make a huge difference to the useful rev range, I guess.

tim

[the poi]

Detonation is possible on a diesel motor--detonation is a physical term describing combustion exceeding mach 1. Internal combustion engines actually experience high-speed deflagration, not detonation. When detonation occurs inside an engine, the whole events happens so ridiculously fast that the pistons doesn't have time to move--thats what causes damage, the explosive force can't be converted to mechanical force. If you inject diesel into a too-high-pressure cylinder, I don't see why it wouldn't detonate instead of deflagrate. Granted, I've never heard of detonation in a diesel, so it may have something to do with the density of the charge, or the detonation resistance of diesel itself, but then, it just might explain why the static CR is lower on them high boost oil burners.