I have a few car related questions...

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Paganisterr
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I have been curious to know the answers for a while now. Finally I've managed to remember and write them down to post them all in here:
  • Why do diesel engines have to be made of Steel?
  • Why do they have to be Turbocharged instead of Supercharged?
  • Recently more and more cars have been having varied maximum torque outputs - say for example 170lb ft at 1400-3500rpm. How does the engine work out whether to output the torque at 1400rpm or at 3500rpm? Throttle-usage? The speed you're going?
  • Why do engines have a usual idle of around 700rpm?
Thanks!
 
Diesel engines are steel because of the explosions that drive the engine, and the increased torque.

I would suspect they are turbo because of the greater volume of exhaust coming out.

I'm not sure but the area where the torque output is in the rpm range might be the power band.

Engines Idle there because if they idled at 0, they would be off.

PS: I have no idea what i'm talking about. :D
 
Most Diesels have Iron crankcases for strength. The compression ignition cycle has a greater shock loading effect on the crank/pistons/rods than a spark ignition petrol engine. So the parts have to be stronger and heavier, hence diesels don't rev to such a high speed. There are issues with flame front speeds too using heavy oil.

They don't have to be iron, ally can be used if it's designed for the job, but I've not much experience with CI engines. The honda Diesels might be ally.

Most are turbocharged as the technology suits diesl engines better than supercharging, and they are simpler in design/construction.

As for torque, all the figures you read are at wide open throttle. The 170ft/lb at 1400-3500 just refers to the fact that between those speeds the engine is making 170ft/lb of torque at wide open throttle.

Idle speeds are usually 700 or so because that is the speed most car sized engines idle at using the least fuel to remain turning without cutting out. Larger engines idle at lower speeds. The largest engine in the world has a rev-limit of 104rpm, but my 675cc bike idles at about 1200rpm.
 
As for the torque...

Many cars today are able to produce more torque than their drivetrain can handle. For example the Ford Falcon XR6T (Australia). The power is handle well by the clutch, gearbox, diff etc... but the raw torque of the engine would break something if used harshly. Therefore the torque is electronically limited to a peak value for most of its rev range.
 
G.T
I have been curious to know the answers for a while now. Finally I've managed to remember and write them down to post them all in here:
Why do diesel engines have to be made of Steel?
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I wasn't aware they had to be made of steel or iron, but diesel engines typically have much higher compression ratios then petrol engines, which means more stress.

G.T
Why do they have to be Turbocharged instead of Supercharged?
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My guess on this one is that most diesel engines already make good torque at low RPM (superchargers work best for adding torque at low rpm) so a turbocharger is more effective and spools quickly for the higher RPM's That doesn't mean they have to be turbocharged, but thats what probably works best.
 
Sakiale
Why do diesel engines need to be 'charged at all? I thought that they don't use air.
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Diesel engines still have to intake air, if they didn't you wouldn't get any combustion at all. The air is just compressed by the piston coming up, then the fuel is injected atomising it mixing with the compressed air causes the combustion.

Well thats how I think it works, that was like from a video I watched ages ago.
 
Oh yes, that is true, I just got it twisted--Diesel engines DO use air, but dont need the spark that conventional otto 4-stroke engines do, instead igniting by combustion pressure.

edit, thanks backspace
 
Diesels actually use a glow plug like a model airplane engine. It's kind of like a light bulb filament. Each explosion reheats the plug (hence the 'glow'), and then when the air/fuel mix is compressed enough, the glowplug ignites the explosion.

When starting a cold diesel, you typically have to sit with the key on for a few seconds while it electrically heats the plug before you can throw the starter over.
 
Thanks for the detailed explanations, guys.

I stuck question #2 there for the hell of it so I could see what you guys would say. I found out the answer a few days ago in evo magazine because someone else was curious as well. They said there's no reason why a Supercharger shouldn't work, but we use Turbochargers all the time because they work so well with Diesels. Even if a Supercharged diesel was made, the low-end power would be very good, but at the higher-end it wouldn't be as good as a Turbo.

I can see why the asker of the question wanted to know the answer, though, as he was saying how much lag Turbodiesels have lower in the rev range and why a Supercharger isn't used to get rid of that.
 
G.T
Thanks for the detailed explanations, guys.

I stuck question #2 there for the hell of it so I could see what you guys would say. I found out the answer a few days ago in evo magazine because someone else was curious as well. They said there's no reason why a Supercharger shouldn't work, but we use Turbochargers all the time because they work so well with Diesels. Even if a Supercharged diesel was made, the low-end power would be very good, but at the higher-end it wouldn't be as good as a Turbo.

I can see why the asker of the question wanted to know the answer, though, as he was saying how much lag Turbodiesels have lower in the rev range and why a Supercharger isn't used to get rid of that.
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One of the reasons why most Diesel engines are turbocharged these days is because it's more easy to turbocharge them than a petrol engine since they have Direct Injection.
When the intake valve on a Diesel engine opens the only thing that is entering the cylinder is air, unlike a petrol engine in wich it needs to already have the fuel/air mixed prior to enter the cylinder. Within the Diesel engine the air goes in - gets compressed (at a very higher rate than in a petrol engine) - the fuel itself is "direct injected" into the chamber - the whole thing explodes... So if you know how turbos work you'll know that they compress only air to be sent into to the engine, hence why it is easier to turbocharge a Diesel engine.
As to why they do it? Well because there is a great Diesel fever (at least in Europe) and every manufacterer is trying to squeeze performance out of their Diesel engines. The famous Merc 190D isn't turbocharged. I'm guessing that modern Diesel have more performance but they won't last as long as the old-school Diesels.
Petrol engines are becoming direct injected to. You have for example FSI from VAG or D4 from Toyota. Also the new GS450h uses direct or un-direct injection as it sees fit... how this works and why I do not know.

I hope that wasn't too confusing!! I have trouble explaining technical stuff in a foreign language....
 
Duke
When starting a cold diesel, you typically have to sit with the key on for a few seconds while it electrically heats the plug before you can throw the starter over.
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Yep, just enough time to put my seatbelt on. Some modern diesels glow the plugs when you turn the alarm off or when you unlock the doors so they're already warm when you put the key in.
 
Hey, it's my turn to see how many I get right. ANd without looking!

  1. I'd guess diesel blocks are made from steel because of the great strength. Durability is what they're after. Steel also holds heat very well, which is a plus when it comes time to start the thing.
  2. Diesel engines don't have to be turbocharged rather than supercharged. They can be supercharged. Or they can be naturally aspirated--they all once were--and there are even some that are turboed and blown. The reason for the forced induction is the need for oxygen. The stroke of a diesel engine is so long that it can't physically suck enough air through the intake to satisfy the fuel's need for clean combustion. You have to realize that even though the RPM of a diesel engine is very low compared to most gas engines the piston's speed is actually much higher because it has so far to travel. Forced induction helps cram tons of air into the chamber to burn the fuel more thouroughly and get rid of all that carbon smoke. On a side note, big turbos (think twice the size of your head, sometimes literally) have bif lag, and the supercharger on the combo engine gets tid of that at low rpm or during initial throttle application.
  3. I see the torque number you posted is that of the Rabbit's new 2.5L I5. That engine has an iron block, a heavy rotating assembly, a massive flywheel, and thus churns huge torque and revs slowly. The RPMs you posted comprise a range--not individual peaks--in which the engine holds its peak torque of 170 lb-ft (speaking of that, the correct torque symbol, in the US anyway, is lb-ft, with a dash and lb first). This car would be very driveable and peppy at low speeds and would hold its speed going up hills easily.
  4. Most engines idle at ~650-900 RPM. It's just physics, and it's what works best. It actually puts less stress on the engine than a lower RPM because vibrations would that much smoother. Obviously it's good for keeping the engine running. The engine makes enough power at that RPM to catch itself when the revs drop from high to idle, like when you're revving at all your buddies in your new car. I know a few people with carbs whos idle was wrong, and the car would sputter and stall every time he revved it because they dropped too low. Another note, My B16 revs down as low as ~250 occasionally, and I have no clue how it does it. It runs as smoothly as a Renesis. Honda really does work magic.

So did I get everything?
 
Duke
Diesels actually use a glow plug like a model airplane engine. It's kind of like a light bulb filament. Each explosion reheats the plug (hence the 'glow'), and then when the air/fuel mix is compressed enough, the glowplug ignites the explosion.

When starting a cold diesel, you typically have to sit with the key on for a few seconds while it electrically heats the plug before you can throw the starter over.
Click to expand...

INteresting, learned something new today! :)👍 +rep, Duke. As if you need it.
 
Yeah, diesels don't use spark. That's the main difference between them and gas engines. Not only do you have to wait for the glow plugs to heat up and keep ingnition going until the motor gets to operating temperature, but on top of that some engines get plugged in for the night when it's really cold outside.
Diesel turns to sludge when it's really cold, so the injectors and fuel rails are kept warm over night with electricity. You'll probably see some peoples' trucks with an extension cord hanging out of it when it's cold. The fuel has to be thawed so it doesn't clog the injectors and such.
 
I knew that they didn't use spark plugs, but I thought the fuel was ignited by pressure alone, wasnt aware of a 'glow plug'.

Since I for some reason can't give rep to Duke OR Touring Mars, keef gets some :)
 
There is a lot of misinformation in this thread.

Superchargers are much less effecient than a turbocharger at low rpm. A supercharger does have off-idle boost, but the boost in a supercharger is directly related to the rpm of the engine. Boost is very low and thus power gains are very low on low rpm. While the boost is there instantly, there's not much when it comes.
Turbochargers are spun by exhaust gases, so there isn't very much boost right off idle as there is very little gas being processed by the engine. However when the gas comes, which it does, the boost is there. Of course there are engines like a 996 GT2 where full boost won't be hit until 3500rpm, but different turbo applications can be chosen for a diesel that will hit full boost much more quickly. Once it does hit full boost, you have much more torque available in the lower rpm.
 
keef
Yeah, diesels don't use spark. That's the main difference between them and gas engines. Not only do you have to wait for the glow plugs to heat up and keep ingnition going until the motor gets to operating temperature, but on top of that some engines get plugged in for the night when it's really cold outside.
Diesel turns to sludge when it's really cold, so the injectors and fuel rails are kept warm over night with electricity. You'll probably see some peoples' trucks with an extension cord hanging out of it when it's cold. The fuel has to be thawed so it doesn't clog the injectors and such.
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You make it sound like you've got ages waiting, it's a 3 or 4 second job heating the glow plugs to the right temperature. And once you've turned the ignition key you go, there's no more waiting after that. As for plugging the engine in, I haven't known anyone in the UK who's ever had to keep their diesel warm because it's too cold.
 
I've seen many heavy-duty trucks that are plugged in. No cars, just the big pickups, and only during the winter. It's a common sight in my area.
 
KurtisGSXR
but my 675cc bike idles at about 1200rpm.
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My brother's 1985 MR2 with our freshly rebuilt engine idles at exactly 1150rpm. I hear this is normal on the MR2 forums...that's a sickly high idle. My Camaro idles at about 550rpm, granted that's with over 215k miles on that old engine. :sly:
 
There's nothing wrong with a higher idle. It uses a bit more fuel, yes, but it won't hurt the engine.

Modern engines are better when spun up a bit, revving them won't hurt them half as much as lugging them. And anything in good condition made after the seventies is applicable as a modern engine.
 

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