Large diesel fuel economy

[Bobux]

Originally Posted by mort

Hi Bobux,
On a Diesel, especially something like you have, a turbocharger will improve efficiency substantially. At high power the Diesel engine is injecting fuel through most of the power stroke, and that fuel burns slowely so the equivalent expansion ratio is quite low. For a compression ratio of 16:1 at full power, (but not black smoke - does it do that?) the expansion ratio is about 4:1. Although the mechanical efficiency of the turbine is lower than the piston engine, there is a lot of power there. Using that power to increase combustion pressure and temperature is like increasing the compression ratio. So you waste less and improve thermodynamic efficiency win - win. Right up to the point that something blows out...
Roughly, the thermodynamic efficiency of a Diesel with 16:1 CR and 4:1 ER is 53% Increase the ER to 15:1 and efficiency goes to 66%

-mort

Can I ask for an explanation here? I am not an expert in thermodinamics, but I came to a different conclusion. I thought of it like this:

Example 1:
Assume I had an engine working under athmospheric pressure, CR of 10:1. When the piston reaches it's top position I have 10atm pressure. Burning all air in the chamber the pressure will rise to 40atm because the air will try to expand 4:1. This way extra 30atm will push the piston down and generate torque.

Then I will shave the cylinder head and raise the compression to 100:1. Initially, (about) same amount of air will enter the chamber and be compressed to 100atm. Burning all air pressure will rise to 400atm because the expansions is still 4:1. Extra 300atm generate torque burning same amount of fuel, which is much more, so efficiency will obviously rise.

Example 2:
Same naturally aspirated engine as before, CR of 10:1. This time I add a turbocharger with ideal intercooler (intake air will be at same temperature). The charger will compress air 10:1, so 10 times more air will enter the chamber, combustion pressure will be again 100atm.
Injecting same amount of fuel I will be able to burn 1/10 of air in chamber, and it will expand 4:1. Once the pressure (and/or temperature) redistributes equally in chamber we will get 1/10 * 4:1 * 100atm + 9/10 * 1:1 * 100atm = 130atm. That is same 30atm extra pressure as in case of burning this amount of fuel without the turbocharger.

Conclusion:
Higher compression does increase torque "pulled" out of given amount of fuel. Turbocharging itself (not counting side effects like temperature change and change in fuel distribution in chamber) does not increase efficiency, but only pushes more air through the engine resulting in higher engine potential (ability to burn more fuel).

[Bobux]

Originally Posted by drmiller100

so what kind of mileage do you expect? Remove half the tires, drive it down the freeway at reasonable speeds, and you will probably see something as high 7 or 8 like every other semi truck out there running empty.

if you want better mileage, raise the compression. raise the fuel pressure, and update the injectors to something from at least the 80's or so.

Originally Posted by bandit86

Your problems are tires; They are low pressure to float over terrain ecause it is a pain to offroad 14 tons without sinking. Impossible. Get highway truck tires and maybe rims and you will see huge improvements. Also the old engine won't help, takes a lot of power to drive 4 differentials, huge transmission. A smaller engine would save you 2000 pounds

Agreed, tires and the drivetrain have the most parasitic losses, that's the part I will focus on. The engine will still be mostly inefficient, so probably the best way to raise efficiency is swapping it for something smaller and modern (+alter the final drive ratio). Less weight will also support it's ability to swim

Originally Posted by Diesel_Dave

I suppose we should also ask what are you actually doing with this truck. That may infulence things too. I assume you're not driving this back and forth to work. Are you hauling stuff with it? On highway? Off road? Towing?

It's a project, so it will not see much use as a one-person-with-a-laptop or a heavy load transport. It will mostly carry it's own 14 tons, sometimes off-road, sometimes off-shore, but mainly on-road use is expected.

[Diesel_Dave]

Originally Posted by Bobux

Can I ask for an explanation here? I am not an expert in thermodinamics, but I came to a different conclusion. I thought of it like this:

Example 1:
Assume I had an engine working under athmospheric pressure, CR of 10:1. When the piston reaches it's top position I have 10atm pressure. Burning all air in the chamber the pressure will rise to 40atm because the air will try to expand 4:1. This way extra 30atm will push the piston down and generate torque.

Then I will shave the cylinder head and raise the compression to 100:1. Initially, (about) same amount of air will enter the chamber and be compressed to 100atm. Burning all air pressure will rise to 400atm because the expansions is still 4:1. Extra 300atm generate torque burning same amount of fuel, which is much more, so efficiency will obviously rise.

Example 2:
Same naturally aspirated engine as before, CR of 10:1. This time I add a turbocharger with ideal intercooler (intake air will be at same temperature). The charger will compress air 10:1, so 10 times more air will enter the chamber, combustion pressure will be again 100atm.
Injecting same amount of fuel I will be able to burn 1/10 of air in chamber, and it will expand 4:1. Once the pressure (and/or temperature) redistributes equally in chamber we will get 1/10 * 4:1 * 100atm + 9/10 * 1:1 * 100atm = 130atm. That is same 30atm extra pressure as in case of burning this amount of fuel without the turbocharger.

Conclusion:
Higher compression does increase torque "pulled" out of given amount of fuel. Turbocharging itself (not counting side effects like temperature change and change in fuel distribution in chamber) does not increase efficiency, but only pushes more air through the engine resulting in higher engine potential (ability to burn more fuel).

As far as the turbo goes, the detailed thermo is complicated, but here's a simple explaination of where some of the advantage comes from:

Without the turbo, your intake pressure is 1 atm, and let's say with a turbo you get to an intake pressure of 3 atm. You'll have roughly 3 times the air at the same engine speed, 3x the air means ~3x the fuel means ~3x the power at the same speed. Without the turbo, you'd have to increase the engine speed ~3x in order to get the same amount of air (and power).

So with the turbo, the lower speed means lower friction losses & lower pumping losses. The heat loss to the cylinder walls should also be less because the ratio of fuel burned to surface area has increased.

[oil pan 4]

Turbochargers normally increase fuel economy on diesels by 10% to 20%.

[mort]

Originally Posted by Bobux

Can I ask for an explanation here? I am not an expert in thermodinamics, but I came to a different conclusion. I thought of it like this:

Example 1:
Assume I had an engine working under athmospheric pressure, CR of 10:1. When the piston reaches it's top position I have 10atm pressure. Burning all air in the chamber the pressure will rise to 40atm because the air will try to expand 4:1. This way extra 30atm will push the piston down and generate torque.

Hi Bobux,
There seems to be a misunderstanding of the meaning of expansion ratio. A real Diesel engine doesn't work exactly like the theoretical "ideal" cycle, but it isn't any better.

In an ideal Diesel cycle the air is compressed, as you say, 10:1. The pressure goes to 10 atm (and a little more due to compression heating) V1 to V2. About TDC the injector adds fuel, which burns and releases heat at exactly the rate that the volume is increasing, so the pressure stays about 10 atm. The injector adds fuel from b to c. Then as the volume continues to expand the pressure and temperature fall until the piston reaches BDC, point d. The expansion from when the fuel stopped burning at c until BDC at d is the expansion ratio, V3 to V1. At low power the b-c distance will be small and the expansion ratio will be almost 10:1. At full power b-c will be most of the 90 degrees of the power stroke and the expansion ratio will be small. As b-c gets longer and the expansion ratio gets smaller the pressure in the cylinder when the exhaust valve opens is higher. The pressure and temperature in the exhaust when the valve opens is lost, wasted power.

If you add a turbine to the exhaust you can recover that power by expanding the pressure and temperature the rest of the way down to 1 atm. You can use the exhaust power recovered by the turbine to spin a supercharger or compressor and pre-compress the intake air. Adding more air to the intake allows more fuel to be burnt increasing the power output, but ignoring that, the higher compression pressure produces a higher compression temperature. Efficiency can be computed as the high temperature minus the ejection temperature all divided by the high temperature. Including a turbine in the exhaust so the final ejection temperature is the same for different operating conditions, you can see that the supercharger acts just like increasing the CR and increases efficiency. The temperature rise due to compressing a gas is the initial temperature multiplied by the compression ratio raised to the power of g-1, where g stands for the ratio of specific heats for the gas. g = 1.4 for air. So if the incoming air is about 80 F (300 K) and the CR is 10, the compression temperature is 753 K. If the equivalent CR is raised to 20:1, then the compression temperature goes to 994 K If the ejection temperature is 373K (212 F) for both cases, the efficiency for CR = 10:1 is about 50%, and for CR = 20:1 about 60%

Adding a turbocharger is expensive and complicated, but if you boost intake pressure by 1 atm you'd improve fuel economy by 20%
-mort

[bandit86]

Gas engines are 10:1. Diesels are 16:1, some as high as 22:1. My Unimog does not have glow plugs or air heater to get I started, I believe it is 18:1. Older army trucks like yours will be set up for crappy fuels, you can adjust your injector timing for better performance. Get a remote mounted turbo, not as efficient as one at a manifold but 10x easier to install.

So gas engine, wide open at best sucks in 1atm of air, compresses to 10atm, fuel ignited and burns and you have 40atm pressure driving the piston down

With turbo, wide open at 15psi boost or 1atm you have 2 ATM in cylinder. Compressed to 10:1 that is 20, x4 and 80atm pressure driving down. Stock old diesels don't like more than 1

But the above is for gas, need to know your compression ratio. At least 15:1, probably closer to 18, but it could be 20

With boost you get high compression when you need it. With a high cr ratio engine, like 100:1 you will neve start it. You will need a starter ten times more powerful than you have. The connecting rods will not take the constant extra load, neither will your pistons.


If the engine is similar, find a forum and see what mods te truckers use. I know my Benz unimog there is tons of stuff I could do to it, nozzles, turbo manifolds readily availible too

The reason why it is more efficient is you have more oxygen availible for the given fuel to ensure a complete burn, at lower rpm you make more power. at 1000rpm it takes 1/2 power to keep the engine turning then 2000rpm, 2000rpm is half power to turn than 4000rpm. if with a turbo you can make same power at 2000rpm as 4000rpm you have increased fuel economy, probably not 2 fold but I bet by at least half that as the mechanical losses are the same. either way a turbo is good, I wish I had one

your gear ratios are probably goin gto suck too, too low and your engine revs too high wasting power, too low gears and it struggles to get going.

my truck was geard for 75km/h top speed. I installed 15% larger tires (stock were 40"dia) and now at 75km/h the engine is not running full out.

picture of mine, 46inch tall tires, 15 inches wide, gets 15mpg

[oil pan 4]

Thats impressive if orange swamp buggy was gas powered I would expect it to get like 6mpg or less.

[gone-ot]

...a diesel's efficiency stems from its high compression ratio (cr) and a turbocharger increases its static mechanical-compression ratio to an even higher dynamic effective-compression ratio, when the turbo is spooled up.

[drmiller100]

Originally Posted by Old Tele man

...a diesel's efficiency stems from its high compression ratio (cr) and a turbocharger increases its static mechanical-compression ratio to an even higher dynamic effective-compression ratio, when the turbo is spooled up.

mods, can we move this to the unicorn fart area???

thanks.

[drmiller100]

Originally Posted by Diesel_Dave

So with the turbo, the lower speed means lower friction losses & lower pumping losses. The heat loss to the cylinder walls should also be less because the ratio of fuel burned to surface area has increased.

you pump three times the air, you have 3 times the pumping losses with a diesel.

friction is mouse nuts in terms of efficiency (mouse nuts are so small as to be really hard to measure).

Diesels have a slow burn rate, so they can't turn high RPM. To make a diesel compete with a gasoline engine, the only way to keep the weight of the engine similar is to put a turbo on the diesel.