Large diesel fuel economy
 

Hi there! I have a pending project - rebuilding a 50-year old army truck. Actually, it's an amphibious 8x8 armored personnel carrier, but it's not the point.

The machine weights 14tons and is equipped with a 12-liter V8 engine, naturally aspirated direct injection diesel. Compression ratio of 16.5, OHV, peak torque 550Nm @ 1200RPM, peak power 180hp @ 2000RPM. Engine is free-flow: no intake restrictions, no turbo, no catalyst.

Because the engine was designed for military purposes it runs almost any fuel mixtures from kerosene to vodka :) but suffers from poor fuel economy: around 70l/100km (about 3MPG) depending on fuel quality.

So the question is: how does one make such engine more efficient?

Subquestions:
- How would compression ratio change affect fuel economy?
- How does cylinder unitary capacity affect fuel economy?
- What is the main drawback of such engine - electronic injection should get it 15% efficiency, what else?
- What are other differences between this one and modern engines? We can assume Ford Power Stroke 6.4 as a good replacement option (electronic injection, twin turbo, exhaust gas recycling, catalyst, intercooled and aftercooled).

Other thoughts: engine and chassis lubricants, tires and lights will all be replace by modern ones.

Can you tell us what it's called? I was thinking BTR-60 but the specs didn't match.

It's OT-64. It runs a Tatra 928-14 engine, which is, phenomenally, almost identical to later Tatra truck engines.

More compression is better.

Gearing is always a fuel ecomomy killer but on an 8x8 I don't think there is much you can do.

Water mist injection would help some.

EGR and catalyst hurt fuel economy.
EGR only helps by warming up the engine faster, beyond that its no good.

By far the best thing you could do is stick a turbocharger on there or install a more modern diesel.

Increased compression ratio - EcoModder
Turbo charging - EcoModder
Water injection - EcoModder

Sounds like you're dealing with a dinosaur. In my opinion, you'd be miles ahead by swaping to a much smaller modern turbodiesel, rather than trying to modify the existing engine.

Consider the fact that you can get a EuroV Cummins ISB 4.5L engine (4 cylinder version of the 6 cyle 6.7L engine that I have in my truck) with the following specs:
Peak torque=760 Nm @ 1400rpm
Peak power=204 hp @ 2300 rpm

Thats more power and toque from an engine with under 40% the displacement. That particular engine is also sold with a single turbo and no EGR. The EuroV engine is sold with an SCR cataylst.

Good to know, thanks ;)

I guess this only applies to forced-induction engines because intake temperatures are higher even after an intercooler.

Again, good to know, thanks :thumbup:

A turbocharger would definitely improve engine characteristics in means of power, it was the first idea that came into my mind when I thought of engine tune-up. But in this case I'd like to focus on fuel efficiency rather then on power (it does not mean I am willing to sacrifice any, it's just not a #1 priority). I guess a turbocharger will not help efficiency much, right?

As said before, it's a military vehicle, it was designed that way. Take a look at HMMWVs - they output 190hp out of 6.5l AFTER turbocharging, and don't have too much torque either. These engines were meant to work with sand in the intake, water in fuel and 1/3 of oil they usually run with... So, it's not the age we are dealing with.

It's fairly obvious that fitting in a complete modern engine will get the best results. I was just wondering why this particular engine consumed that much and what the consumption would be with a different engine.

sir, your vehicle weights 14 tons. it has 8 tires on the ground, all of which have axles, drivelines, and lots of steering knuckles.

the tires are bias ply. I bet the mileage numbers you quote are overland offroad and not on the highway.

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.

after that, diesel is diesel - you are not going to see a huge difference.

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

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?

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