Automotive 400 HP gasoline pickup engine vs 400 HP semi engine

[JDizzy09]

If you took a 400 horsepower gasoline pickup engine and a 400 horse power semi engine and hook them up to a transmission so that they had the same output rpm while each engine is at their peak horsepower rpm would they be able to pull the same load fairly similar?

 

[davenn]

If you took a 400 horsepower gasoline pickup engine and a 400 horse power semi engine and hook them up to a transmission so that they had the same output rpm while each engine is at their peak horsepower rpm would they be able to pull the same load fairly similar?

what do you think and why ?

 

[JDizzy09]

I think it would be exactly the same if the power band on a graph was the same shape on its way to reaching peak horsepower even though the stretch of rpm would be a lot higher on the gas engine. I think that because horsepower is power applied over time which makes it seem simple that it would be the same. The reason I want to confirm that with people who would really know is because it seems slightly odd that semi's would give up all that potential horsepower that they could get just by raising the peak torque by 500 RPM just for the purpose of having less wear and tear by keeping it at a very low RPM.

 

[JDizzy09]

*it seems slightly odd that they would want to produce that much torque for such a relatively low amount of horsepower just for the sake of saving on wear and tear.

 

[JDizzy09]

So basically I'm wondering if there's any benefit to having that much torque from a semi engine (vs say the relatively low torque you'd get from a 5.7 hemi with the same horse power) other than durability.

 

[jbriggs444]

I think it would be exactly the same if the power band on a graph was the same shape on its way to reaching peak horsepower even though the stretch of rpm would be a lot higher on the gas engine.

In your original post, you asked about "pulling the same load". There was no mention about accelerating up to speed. So let us focus the question. Are you talking about:
A. A gasoline engine and a diesel engine both pulling the same load uphill at maximum power?
B. A gasoline engine and a diesel engine both cruising down the highway at the posted speed limit?
C. A gasoline engine and a diesel engine accelerating from a standing start without the benefit of continuously variable transmissions?

In each case, what metrics of goodness are you looking for? Fuel economy, performance, wear, smoothness of power, resulting pollution?

 

[sophiecentaur]

In your original post, you asked about "pulling the same load". There was no mention about accelerating up to speed. So let us focus the question. Are you talking about:
A. A gasoline engine and a diesel engine both pulling the same load uphill at maximum power?
B. A gasoline engine and a diesel engine both cruising down the highway at the posted speed limit?
C. A gasoline engine and a diesel engine accelerating from a standing start without the benefit of continuously variable transmissions?

In each case, what metrics of goodness are you looking for? Fuel economy, performance, wear, smoothness of power, resulting pollution?

It strikes me that the problem is probably not about an automotive application. "Hooking things up" to a load would suggest a static machine that uses a lot of power - like a pump, crane or plough (original use of static traction engines).
I do wish that people would include more context and details about their initial questions. It can save such a lot of wasted questions and answers. Otoh, I can sympathise when the reflex reaction of PF is to think of a 'better way' of doing something.

 

[sandy stone]

My take is that the question was more of a theoretical nature, regarding the relationship between torque, engine speed, and horsepower (and maybe, durability).
Automotive horsepower is more applicable to top-end speed, which is not really what you're looking for in a semi. Instead, you obviously want durability, since these trucks run the highways for untold hours and downtime is a bad thing. You also want torque, in order to get a heavy load up to speed or climb an incline, and low-end torque (broad torque peak) helps for smoother operation on the road. So if both engines have the same peak torque, it makes sense to peak at a lower RPM in the semi and give up some horsepower.

 

[JDizzy09]

Ok I'll try to be more specific while simplifying. If you had 2 identical semi's, and both were carrying a 30 ton load. But one had a regular 400hp diesel engine you'd see in a semi with it's standard transmission, the other had a 5.7 liter 400hp engine with only 350 lb-ft torque and a transmission specially equipped so that you could get both to be at their peak horsepower (400) at 70mph. Then while they're both going 70mph side by side on a flat road they hit 25 degree incline at the same time. Would one slow down more than the other?

 

[jbriggs444]

Ok I'll try to be more specific while simplifying. If you had 2 identical semi's, and both were carrying a 30 ton load. But one had a regular 400hp diesel engine you'd see in a semi with it's standard transmission, the other had a 5.7 liter 400hp engine with only 350 lb-ft torque and a transmission specially equipped so that you could get both to be at their peak horsepower (400) at 70mph. Then while they're both going 70mph side by side on a flat road they hit 25 degree incline at the same time. Would one slow down more than the other?

25 degrees. Holy mountaineering, Batman! A ten percent grade is "steep". A 25 degree incline is a forty-six percent grade. That's beyond anything you'll be driving up in a semi.

The two engines will have some small effect from their kinetic energy of rotation. But by comparison to a 30 ton load going up a 25 degree incline, that's going to be pretty negligible. Let's set up some equations -- how fast can 400 horsepower raise a 30 ton load up a 25 degree incline?

A horsepower is approximately 750 watts. We have 400 of them, so that is about 300 kilowatts.

We have 30 tons (gross vehicle weight?). That is 60,000 pounds. At 2.2 kg per pound, that is about 27,000 kg. Multiply by the acceleration of gravity (call it 10 meters/sec²) and we are talking about 270,000 Newtons.

Divide 300 kilowatts by 270,000 Newtons and you have around 1.1 meters per second. If you work out the conversion and unless I've mucked something up [guilty!], that's about 2.4 miles per hour. Irrespective of engine type.

Edit: Apologies to @JDizzy09, I forgot to account for the slope. That 2.4 miles per hour is the rate of vertical rise. The rate of diagonal run would be that number divided by the sine of 25 degrees. You could actually make 5.6 miles per hour up a 25 degree slope.

A more reasonable 10 percent grade is a 5.7 degree slope angle. You convert percent grade to slope angle by taking the inverse tangent. The inverse tangent of 0.10 is 5.7 degrees. By no coincidence, the sine of 5.7 degrees is just about 0.1 [The sine and the tangent of small angles are nearly identical]. You could drive that semi up a ten percent grade at about 24 miles per hour.

 

[JDizzy09]

Oh dang. I appreciate the quick math, that's honestly impressive. But what you're saying then is the answer is that they would slow down at the same rate?

 

[russ_watters]

Oh dang. I appreciate the quick math, that's honestly impressive. But what you're saying then is the answer is that they would slow down at the same rate?

Yes, but that assumes no losses. Due to the larger reduction ratio, the gas engine's losses will be much higher than the diesel's.

You also aren't factoring efficiency into your logic, whichvyou will need to if you want a real world answer to your base question of why diesels are used instead of gas engines in trucks.

 

[JDizzy09]

Ok, yeah in that imaginary scenario for me there were no differences in power reduction to the wheels due to the amount of gear teeth meshing per minute between the two or anything else like that, i was just wondering if horsepower truly is simply horsepower regardless of what the combination of factors are to get it (rpm and torque) and that answered it so thanks guys. This is my first time posting a forum question, so sorry for my nonsense way of asking it. Lol

 

[MRFMengineer]

Horsepower is horsepower, this is really more of a duty cycle question. For example the semi engine is designed to put out 400 HP mile after mile, day in day out. The Hemi gas motor is much ligher duty, only designed to put out the 400 HP for accelerating onto the freeway or a drag race. You would not want to hook up a trailer and head up a mountain pass asking for 400 HP, would reduce its life.

Also, in the marine diesel world, you can buy the same diesel in a variety of ratings, depending on application, pleasure vs "continuous" ratings. Typically ratings 1-5. The Volvo D13 is rated at 700,800,900 HP for ratings 3,4,5, all in the specs. (also lower ratings)

 

[Dr. Courtney]

Pulling real loads requires accelerating over a range of rpms, so invariably you need to consider all the relevant sections of the HP vs. rpm curve and not just the peaks.

 

[JDizzy09]

Yeah so

Pulling real loads requires accelerating over a range of rpms, so invariably you need to consider all the relevant sections of the HP vs. rpm curve and not just the peaks.

SO I'll give a whole new scenario I think might be better. Take those same 2 engines. Hook them directly up to water pumps that circulate water through identical pools and pipes to get to the pools. Now have both of them pumping their maximum amount of gallons of water per minute that they can handle without bogging down while they're both at there peak horsepower (400). (In this dream scenario both pumps, even though they're going to be running at different rpm and have different amounts of water being pumped per rotation, are themselves going to have the exact same amount of resistance on the engine so that the only resistance on the engines is the amount of water they're pumping.) Will one be able to pump more water than the other before bogging down?

 

[Dr. Courtney]

Yeah so

SO I'll give a whole new scenario I think might be better. Take those same 2 engines. Hook them directly up to water pumps that circulate water through identical pools and pipes to get to the pools. Now have both of them pumping their maximum amount of gallons of water per minute that they can handle without bogging down while they're both at there peak horsepower (400). (In this dream scenario both pumps, even though they're going to be running at different rpm and have different amounts of water being pumped per rotation, are themselves going to have the exact same amount of resistance on the engine so that the only resistance on the engines is the amount of water they're pumping.) Will one be able to pump more water than the other before bogging down?

I don't think so, assuming their HP specs were properly measured and are right in the first place.

But there may be fine details in what you mean by "bogging down".

In performance curves with peaks, the slope of the curve near the peak can be important as variables are changed. Variables are never exactly constant in an experiment. The curve with a broader, flatter peak will tend to resist these changes better than the curve with a narrow sharp peak where changes are more likely to reach the "slippery slope" of the performance curve. Engines with broader, flatter peaks in the performance curves are easier to keep near the top. In real life (or real experiments) keeping the load exactly constant is tricky.

 

[JDizzy09]

I don't think so, assuming their HP specs were properly measured and are right in the first place.

But there may be fine details in what you mean by "bogging down".

In performance curves with peaks, the slope of the curve near the peak can be important as variables are changed. Variables are never exactly constant in an experiment. The curve with a broader, flatter peak will tend to resist these changes better than the curve with a narrow sharp peak where changes are more likely to reach the "slippery slope" of the performance curve. Engines with broader, flatter peaks in the performance curves are easier to keep near the top. In real life (or real experiments) keeping the load exactly constant is tricky.

Ah perfection! You answered the question I didn't know how to ask. I knew there was some other reason to use the engines that they do in semi's other than just to keep a low RPM for wear and tear purposes, just didn't know what it was. That explains it. Very good thank you.

 

[Jac5522]

I really enjoy these theoretical explorations. Engines are generally applied to their purposes based on peak torque not horsepower. A 400 horsepower 800 cubic inch inline 6 turbo diesel truck engine would have 3-4 times the torque of the average 400 hp pickup truck engine. It would lope along at 1500 rpms for 2 million miles, get overhauled and do it again. The 400 hp gas pickup engine would have been recycled at 200k miles and could not pull an 80,000 pound truck for a week without burning up.