honda_pilot

My parents drive a 2000 VW Jetta TDI, its a diesel model. It has only 90 hp! But 162 ft/lbs... why does this occur in diesel engines? Thats almost double torque over horsepower.

Its got a factory turbo, and you can feel spool up time, but for 90 hp, it pulls alright. Just goes to show how it all really comes down to torque.

Sorry its not at all related to Hondas but the question has been sitting on my mind, and I don't know **** about diesel engines. Thanks.

MrFatbooty

Diesel motors generally have turbos. They make lots of torque at low RPM but don't rev that high.

HP = (RPM * lb/ft) / 5250

So if you have a lot of torque but not a lot of RPM, you end up with not a lot of horsepower.

Slow-N-Low

Diesels make gobs of torque because they have a long stroke. The longer stroke gives more leverage, hence more torque. The reason why they make relatively little horsepower is because they are pretty much done by 5250 RPM. Turbos help horsepower a little, but they're used mainly for scavenging, not power. At least that's how it is with the big ones.

Gearhead

I don't think turbos are for "scavenging" in the traditional sense, or any other sense I know of. I could be wrong, but turbos make back pressure. The added boost makes torque, which equals HP.

Slow-N-Low

Remember that diesels already operate at very high cylinder pressures. You can't just keep increasing the pressure without disastrous results. But you can make the best use of the volume that gets compressed. Scavenging means that instead of having 80 or 90% air in your compression stroke, you can get close to 100%. More air, more oxygen. More oxygen, more fuel. More fuel, more power. I know it sounds counterintuitive, but that's what my power mechanics teacher said.

Gearhead

80 or 90% air?? what would the other 10-20% be if it weren't air? Typically scavenging is referring to removal of exhaust gas using a chain of exhaust gas pulses that are tuned. Turbos are not going to help scavenging in that manner. They create positive pressure in the manifold. You can't get something for nothing.. it's the positive pressure in the exhaust manifold caused by the actual flow of the exhaust, and the continous expansion of exhaust gas that makes this pressure. I have a 7.3 powerstroke. I know it makes boost because my boost gauge reads around 20PSI when I'm on it. If you check the exhaust backpressure sensor reading, it's always about 50% higher than the boost pressure. I don't know of any overlap in the valves, so the forced evacuation theory only holds true on the old Detroit 2 stroke diesels that had a supercharger for that. I might be wrong.. it's happened b4, but I'm pretty sure of this.
Don.

Slow-N-Low

The other 10-20% is spent exhaust gasses. In the real world, the exhaust stroke does not remove all of the exhaust gasses. Yes, scavenging refers to the removal of these exhaust gasses. No, it doesn't refer to only one technique.

No offense, but I think you need to review the Otto cycle basics again. The gas expansion phase happens during the power stroke (3). During the exhaust stroke (4), the piston pushes the spent gasses out into the exhaust manifold.

Turbo scavenging happens during the overlap period, where both the intake and exhaust valves are partially open. Valve overlap is used to help scavenging and to promote cylinder filling. High-RPM motors use more overlap to make up for the decreased duration at speed. But since Diesels are low-RPM motors, there's relatively little overlap. That's where the turbo comes in, blowing more air in during the shorter overlap.

The fluid mechanics of an internal combustion motor are exceedingly complex. A couple of scalar meter readings simply can't describe everything that's going on.

Gearhead

So, you're telling me that the turbo is making more power because the lower pressure it creates in the intake is aiding exhaust out of the higher pressure exhaust manifold? I'm seriously having trouble believing that the 1.3BAR (2.3 times the normal air charge) is not what is making the power, but the 20% less exhaust dilution is. I'm not saying that less dilution isn't making a little more power, but I feel most of the power is made by the denser charge. I know my truck adds fuel according to boost. If the turbo were for scavenging, it wouldn't do this (If so, please explain why).
Another question, why would a truck manufacturer go throught the trouble of intercooling if they were just looking to scavenge?
1 More: Why is it, when a guy cranks up the boost on a 7.3 powerstroke to say... 30PSI it makes 350HP instead of the 225HP it makes at 20PSI. Is it really getting rid of that much more exhaust? Is there enough exhaust left in the combustion chamber to dilute the incoming charge that bad?
Thanks for any info.
Don.

Slow-N-Low

Gearhead, you're still not getting the mechanics of the Otto cycle. I can't explain it to you if you can't understand that there's more going on than just two scalar pressure readings. You're going to have to learn the rudiments of internal combustion engines first. But something tells me that you're in this for your ego, not to have a nice conversation. I don't need any more of that.

Gearhead

You're right. I'm a freakin' moron. When all else fails, insult someone's intelligence. Good answer. I'm finished myself. You must be right. It is amazing that I've made it through all these years without any knowledge of the otto cycle, but I thought we were discussing Diesel cycle engines... though many concepts are the same, there are some variances, but whatever. I will not believe you if you can't provide proof. I've at least provided some silly scalar measurements that I understand to be factual. I would like to learn, but your ego prevents you from backing up your data. I will not be responding to this anymore. Thank you for your time.