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How to compare hp diesel v. gas


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#31 Cvans OFFLINE  

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Posted August 29, 2011 - 11:03 PM

:iagree::yelclap::yelclap::yelclap::yeah_that:

#32 Utah Smitty OFFLINE  

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Posted August 31, 2011 - 06:30 PM

It IS an interesting topic, indeed. A couple other things that come into play in engine torque is the length of stroke, and how fast the fuel burns. Typical diesel engines have a longer stroke than bore diameter. However, the small one-cylinder Yanmar and all of the Kubota diesels I've dealt with have a shorter stroke than bore diameter. I suspect this is what allows them to get 3,600 rpm--vehicle and agricultural diesels tend to stay around 2,000 rpm give or take.

I don't know for sure, but I suspect that diesel fuel burns slower than gas, so you have power lasting longer through the stroke of the piston. The same effect is achieved with high-octane gas or water injection... both slow down the burning rate of the fuel and thus provide more power.

FWIW,

Utah Smitty
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#33 JDBrian OFFLINE  

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Posted September 01, 2011 - 08:50 AM

It IS an interesting topic, indeed. A couple other things that come into play in engine torque is the length of stroke, and how fast the fuel burns. Typical diesel engines have a longer stroke than bore diameter. However, the small one-cylinder Yanmar and all of the Kubota diesels I've dealt with have a shorter stroke than bore diameter. I suspect this is what allows them to get 3,600 rpm--vehicle and agricultural diesels tend to stay around 2,000 rpm give or take.

I don't know for sure, but I suspect that diesel fuel burns slower than gas, so you have power lasting longer through the stroke of the piston. The same effect is achieved with high-octane gas or water injection... both slow down the burning rate of the fuel and thus provide more power.

FWIW,

Utah Smitty


Thanks for the input Smitty. I think I read somewhere that the speed of combustion of diesel limits RPM's to about 5500. I'm not sure if turbocharging or other tricks would influence that. If that is the case, then your theory about a relatively slow burn resulting in a longer push on the piston seems to make sense. It's too bad that diesels are slow to be adopted in cars in North America. On the other hand, the latest EPA restrictions result in the use of Diesel Particulate Filters to meet the standards. I have heard that a number of manufacturers are having trouble with these, notably Subaru's new boxer diesel which has been released in some markets but is having drivability and fuel consumption issues related to the DPF. Volkswagen seems to have their diesels sorted out.
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#34 Chuck_050382 OFFLINE  

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Posted September 01, 2011 - 01:47 PM

Plus smoke from a gas engine is bad. Smoke from a hard pulling diesel is just so cool! :D

:rocker::D

#35 Utah Smitty OFFLINE  

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Posted September 01, 2011 - 04:29 PM

Thanks for the input Smitty. I think I read somewhere that the speed of combustion of diesel limits RPM's to about 5500. I'm not sure if turbocharging or other tricks would influence that. If that is the case, then your theory about a relatively slow burn resulting in a longer push on the piston seems to make sense. It's too bad that diesels are slow to be adopted in cars in North America. On the other hand, the latest EPA restrictions result in the use of Diesel Particulate Filters to meet the standards. I have heard that a number of manufacturers are having trouble with these, notably Subaru's new boxer diesel which has been released in some markets but is having drivability and fuel consumption issues related to the DPF. Volkswagen seems to have their diesels sorted out.


Well, I do know that high octane gas burns slower than lower octane. Piston-engined aircraft in WWII used water injection to accomplish the same thing, although it could only be done for a limited time (per my Step-father who was a Chief Aviation Machinist's Mate and Enlisted Pilot). I studied Diesel Technology in college, but I don't recall if diesel burn rate is a factor in it's torque.

As for turbocharging, it puts more air into the cylinder, thus increasing volumetric efficiency. I live at about 4,500 feet elevation, and naturally aspirated diesels show a decided drip in performance at higher elevations. Turbocharging tends to practically do away with that (that's why my '85 Ford F-250 has an ATS turbo on it!!)

Good discussion going here... I always benefit from other's insights... keep it up.

Utah Smitty
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#36 Boss 448 OFFLINE  

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Posted September 06, 2011 - 10:57 AM

One thing that I think is significant is the fact that a good diesel engine is designed from the ground up to operate under the same conditions (pre-ignition referred to as dieseling in a gasoline engine) that will destroy a gasoline engine in short order. This lesson was lost on GM engineers that tried to build diesel engines based on beefed up gasoline engine blocks in the eighties. The result was the infamous Oldsmobile and Cadillac diesels that were a total failure. In fact the only good use of those beefed up blocks was for NASCAR racers that put them to use as gasoline powered racing engines.

JN
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#37 Utah Smitty OFFLINE  

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Posted September 06, 2011 - 11:28 AM

I remember those... they also had solid cam followers. Some of them would only get 30,000 miles before they needed a new cam, etc. I didn't realize that NASCAR used them, though.
One thought--some of the foreign manufacturers have built diesels based on gas engines--the VW diesel comes to mind. I wonder what they did to get them to work as well as they do?

Steve
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#38 tractorgarden ONLINE  

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Posted September 06, 2011 - 12:18 PM

This is a very interesting topic. I am going to put my 2cents worth in. If I am wrong in anything I write here please correct me. I want to learn more about this topic.
I think there is a bit of a misconception out there about torque and HP. Torque is the amount of force produced by the engine. HP is that force multiplied by a constant and the RPM's. So HP is essentially the product of torque and engine speed. If 2 engines produce the same torque at the same RPM's then the HP will be the same regardless of the type of engine. As others have said, the difference in perceived power has more to do with the shape of the curves. A diesel is usually of a much larger displacement than a gas engine of the same HP. Torque is very dependant on displacement, so a larger engine will generally produce more peak torque. I'm not sure of the reason, but diesels produce peak torque at lower speeds. It is likely to do with the dynamics of diesel combustion. If you were to compare a gas engine and a diesel of equal displacement it would be a much closer contest.
The way I look at it is that when lugging under load, a diesel will almost always have more HP available then a similar rated gas engine. So under those conditions, which is what matters in the real world,the diesel is the higher HP engine and out performs the gas engine. It is HP that does the work.
A diesel is much more efficient than a gas engine at lower power levels. This is mostly to do with the pumping loss in the gas engine. Pumping loss refers to the intake vacuum in a gas engine that is caused by the throttle restricting the flow of air into the engine at small throttle openings. The engine is wasting energy by creating this vacuum. In a diesel the intake is wide open so there is a much smaller vacuum generated. The other major factor in Diesel efficiency is that the fuel has about 15% more energy/volume than gasoline. Even at wide open throttle the gas engine is at a disadvantage of 15%.
I think another factor is that the diesel is designed for continuous duty at high power levels and most of the gas engines used in modern GT's are not. The continuous ratings for a gas engine are much lower than the peak ones used in the spec sheets. The heavier construction of the diesel ensures that it can maintain high power output levels over a long lifetime. It is not uncommon to see diesel tractors with 10,000hrs+ on the clock. Sorry for rambling on!

Thank you Brian, I agree with you. Now lets put a old two-cycle Detroit into the mix, it lowers the displacement per horsepower at the expense of your eardrums. You have to run them like you hate them :mad2:. But the sound effect to power ratio is very large! I still love my old leaktroit's. We are running a 6V92 turned up in our 4x4 e-one rescue pumper at the firedept. Stock engine hp is 350 which works out to aprox. .63 hp per cubic inch. 552 cubic inches of blown and turbo-ed black smoking noise! Our tanker has a big cam 350 Cummins 855 Cubic inch which work out to .41 hp per cubic inch. The Cummins has a much more usable torque band then the Detroit. This is why I love Gttalk, The things we get to bs about ! Shawn
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#39 Cvans OFFLINE  

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Posted September 07, 2011 - 11:23 AM

This has been a very interesting string and I would like to thank everyone for participating. I wish at this late point in my life that I had taken more time when young to have investigated diesels more thoroughly. In the last few years I have come to really appreciate these unique engines. For some reason this country seems to be far behind the rest of the world in our acceptance of diesel as a viable source of power in smaller vehicles.
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#40 JDBrian OFFLINE  

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Posted September 07, 2011 - 03:54 PM

This has been a very interesting string and I would like to thank everyone for participating. I wish at this late point in my life that I had taken more time when young to have investigated diesels more thoroughly. In the last few years I have come to really appreciate these unique engines. For some reason this country seems to be far behind the rest of the world in our acceptance of diesel as a viable source of power in smaller vehicles.


I agree and the EPA regulations make it very hard for companies without diesel experience to get one approved in a passenger car. These new Diesel Particulate Filters are turning out to be an issue in some cars. Volkswagen seems to have theirs sorted out. There is so much technology required to meet the standards that the simple diesel is getting very complex and that is making them less reliable and more costly to build and maintain. My brother is an electrical contractor and ran 2 duramax diesels. Just before the warranty expired, one of them had serious engine trouble that would have cost over 5K to repair. When he looked at all the costs of running and maintaining them he sold both of them and went back to basic gas powered work trucks.
The truck market is where the diesel tech has gone in North America, with ever higher Horsepower and torque to the point where it is getting ridiculous. What the market really needs IMO is a basic work truck like the original Dodge diesels with the Cummings that sounded like a backhoe and only had like 160 HP. A modern version of that truck would be a good work truck for most people, who don't need a 400HP pickup truck.
The europeans are slowly bringing diesel to North America in their luxury cars, but it is going to take a long time for it to trickle down. Meanwhile gas engines are getting more efficient and hybrids are taking the market for high fuel economy. It will be interesting to see what happens in the next 10 years. Will diesel gain ground in the passenger car market or will hybrids become more widely used. Stay tuned!
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#41 Utah Smitty OFFLINE  

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Posted September 07, 2011 - 10:10 PM

I lived in Germany for 4 years in the late 80's. Diesels are very common there--one big thing in their favor is that the fuel is cheaper than gas. I think one of the reasons diesels are having problems being more widely used in the US is that, in Utah at least, diesel fuel is quite a bit more expensive than gasoline--often in the amount of 30 to 40 cents a gallon.

To me this is primarily manipulation of the price. Diesel costs less to refine, it stores longer (as long as you don't get bacterial growth, or waxing in the winter), and, if I remember right, you get more diesel fuel from a barrel of crude oil than gas. Another problem with diesel fuel is the new requirement for low-sulfur fuel. We're finding that there are more problems with injection pumps, etc., esp. on older diesels that are running the new fuel.

I have an '85 F250 4x4 with a 6.9 Cornplanter diesel with the ATS turbo kit. I love to drive the truck... the base engine got about 155 hp to 175 hp... the turbo adds about 50 hp to this. But, even better, the turbo gives better performance for Utah's high altitudes. I just wish it didn't cost $135 to put 38 gallons of fuel in the ol' girl.

U S
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#42 tractorgarden ONLINE  

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Posted September 08, 2011 - 07:08 PM

The fella I am working with now has a 09 3500 Cummins ram running 20% diesel the rest is C.N.G. great performance, great mileage. The technology is here . I think a per gallon equivalent of C.N.G. is around $1.02 in Utah. It is About 2.02 here. Granted he is not running the 600.00 kit for diesels on E-Bay. His was 3000.00 bucks but has interface with the engine management computer. It increases the power big time if you ask it to, but turned down to stock levels it gets great economy. Shawn
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#43 mchi OFFLINE  

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Posted September 08, 2011 - 07:57 PM

One other thing about diesel compared to gas. Diesels can deal with a less than ideal air-fuel ratio. From what I can understand when you get the black smoke it means that it is burning rich, if you add nitrous that problem goes away:). Diesel also idles on very little fuel.
Plus diesels with mechanical fuel injection can burn all kinds of things.... waste veggie oil, its own crank case oil (run-a-way), JetA/B, kerosene.
Up here in Canada it is unusual to see diesel more expensive than gas. I live close to the boarder and in ND diesel is always more than gas, sometimes it is cheaper to fuel up in Canada. They blame it on taxes here, I am not sure what the reasoning would be in ND.
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#44 Utah Smitty OFFLINE  

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Posted September 08, 2011 - 09:20 PM

My kid brother has a 2006 F250 with 6.0??? (I'm not sure). He was thinking of going to a propane injection system... supposedly gets better power etc.

I'd be half tempted to go with the CNG just to get a little more fuel economy. Best I've ever got with the truck was 12.5 mpg (4.10 rear end and driving very conservatively). I put a 3.55 rear end in, and dropped a notch on tire diameter to get an approx. 3.70 ratio, but now the thing only gets about 10 mpg... Maybe it's the tires... their tread is fairly agressive.

U S
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#45 Tennblue59 OFFLINE  

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Posted September 16, 2011 - 10:05 PM

Diesel used to be less expensive than gas here in the states. The prices flip flopped about 4.5 minutes after I bought my 04 Chevy 2500hd duramax!!
But the fuel ecomomy when towing (which it does a lot of) compared to gas motor helps!!

I read somewhere that diesel is more expensive in the US because of the process used in the US to crack fuel into it's components is more efficient at producing gas than diesel - since most of the us market (aside from commercial trucks) used gas whe4n these plants were created. As the market demand for diesel has increased, supply/demand - price goes up for limited supply.

European market uses differently tuned process to produce more diesel, so price is lower.

Course that may just have been hogwash to justify price gouging on diesel fuel.....:wallbanging:
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