Altitude & Heat: gas/petrol vs. diesel engines

[mauricio_28]

At altitude, is it true that a turbo diesel engine will outperform (fuel consumption and power) a naturally aspirated gas/petrol engine? If, for example, a gas/petrol engine gets 15mpg at sea level, what mileage could I expect at 10,000 fasl? Will the turbocharger in a diesel engine compensate for the lower air density at altitude resulting in less of a performace loss?

I've only driven a diesel in the tropics where the lowest night time temperatures are perhaps in the 40s Farenheit. What about starting a diesel engine after a night of, say, zero farenheit temperatures?

 

[michaelgroves]

A turbocharger is ideal, at least in principle, for high altitude. Any engine, petrol or diesel, can be tuned for altitude (or temperature), but unless you live at those high altitudes, it's not really feasible.

A turbo, on the other hand, regulates the manifold pressure on an absolute basis, so changes in altitude shouldn't affect it.

Changes in ambient temperature are not so easy, since a cold air charge contains a greater mass of air than a warm air charge, at the same pressure. So the turbo can't compensate, unless it's computer controlled and the air mass is metered.

 

[ntsqd]

Aftercooling a turbo'd engine will help further minimize the effects of altitude by making the air density & temp fed to the engine more consistent. Though it will never make it always the same regardless of where you are.

I can say that I've driven a turbo'd, but not aftercooled, VW diesel over two 10,000'+ Colorado passes and the engine behaved pretty much as it did at sea level. This with only 6 psi max boost.

 

[mauricio_28]

Is this "aftercooling" performed by an intercooler? If so, it seems that a turbo, intercooled (diesel) engine is superior to a naturally aspirated, non-cooled gas engine at altitude. For an Andean expedition where roads at 13,000 fasl are par for the course, it seems that the choice of engine is clear.

 

[ntsqd]

"Intercooled" is a commonly misapplied term, so yes.

Intercooling is when the heat exchanger is between the turbo and the supercharger.
Aftercooling is when the heat exchanger is between what ever boost method or methods are used (turbocharger, supercharger, ECT, etc.) and the engine.

 

[michaelgroves]

Is this "aftercooling" performed by an intercooler? If so, it seems that a turbo, intercooled (diesel) engine is superior to a naturally aspirated, non-cooled gas engine at altitude. For an Andean expedition where roads at 13,000 fasl are par for the course, it seems that the choice of engine is clear.

Technically, it's not really a fair comparison, since a petrol engine can also be turbo charged (and/or intercooled), and would have all the same advantages. But for reasons other than the high-altitude, I would recommend a diesel engine anyway for what you are suggesting.

The intercooler is probably of lesser importance than the turbo (especially as the air at 4000m is pretty cool to start with!), but it is worth some extra horsepower, other things being equal. Another benefit of an intercooler is that if your turbo happens to dislike the South American oil you're using, and disintegrates, the bits and pieces don't end up in your valves and cylinders!

Diesels can be harder to start than petrol engines, especially in cold climates, but that's certainly not a rule!


(P.S, while ntsqd is absolutely correct about it technically being an aftercooler, the last guy who actually called it an aftercooler did so in about the 1940's! :hehe: )

 

[OS-Aussie]

Intercooling is when the heat exchanger is between the turbo and the supercharger.

between the turbo and the supercharger. :beer: you sure on that one ???

 

[Redline]

I have driven turbocharged diesels at high altitudes and they are terrific. The loss of power is negligible. Many of the modern light-truck turbo diesel are advertised as making 'rated power' up to 10,000-ft. If I had my way everything I own would be a turbo diesel.

My old '96 Power Stroke doesn't have an intercooler/aftercooler, but it still does the job very well. It has pulled a 30-ft travel trailer over Rabbit Ears Pass in CO (9,426-ft) & Sonora Pass in CA (9,624 ft), and many other lesser passes.

 

[mauricio_28]

good to know. Have been pre-planning (read, fantasizing) about a trans-Andean expedition. Based on the info shared here, buying a gas/petrol truck in the US seems like a bad choice. Buying a TDI Hilux in, say, Guatemala or Panama makes a lot more sense.

 

[michaelgroves]

between the turbo and the supercharger. :beer: you sure on that one ???

Yup, strictly, as ntqsd says, intercooling refers to cooling the air charge between compression stages, usually on multi-stage turbines. For cars, though, there have been very few multi-stage systems, though there have been some cars that use both a turbo- and a super-charger. Nowadays, though, intercooling has really just come to mean cooling the air charge between the charger and the inlet manifold.

 

[ntsqd]

between the turbo and the supercharger. :beer: you sure on that one ???

Very sure. P-38J's & later used them as did some (don't think very many, but don't know for sure) of the 2 stroke GMC diesels that had both a turbo and a supercharger. Working in the engine development business we use "aftercooler" to describe what is commonly referred to as an "intercooler." None of our current designs employ an actual intercooler.

 

[GeoRoss]

For what it is worth, all of the vehicles that I have used in Tibet have been gas. That goes for the 2-ton support vehicles too. I have never seen a diesel there, granted I haven't searched them out either. It just seems to be what those folks use there.

We start at about 12K in elevation and go up. The highest I have ever had a vehicle is 16.5K, but friends have had them higher.


Never have found out why there are no diesel cruisers in Tibet though.

 

[R_Lefebvre]

The biggest impediment to a turbocharged gasoline engine for expedition use is fuel quality. You would typically want to use at least 91AKI Octane, which may not be available where you are driving.

One could employ "turbo-normalizing" and still run regular gas, but I've never heard of such a thing on cars. I've seen it on airplanes however. The idea is to only boost the air pressure up to that of sea level, no higher. You aren't increasing the output of the engine, just ensuring it can still make sealevel hp numbers at higher altitudes.

 

[dieselcruiserhead]

I live at about 7500 ft and regularly get up to as high as 9500 or so. With a turbo diesel I notice no loss of power and it performs excellently.

If anyone wants a more technical explanation:

The way all engines work is there is a set amount of fuel and it must have a matching amount of oxygen to adequately match this fuel. This is the case with both gasoline and diesel engines. A turbo charger is a propeller, powered by the force of air exiting your exhaust (so it is close to free power) that rams air into your engine. So it pre-pressurizes the air before the air is compressed in each engine cylinder, it also typically overpressurizes so it provides more air than the combustion process can consume. Diesels differ from gas engines, for some reason, in that the balance of air-fuel can be varable with no negative effects other than actual performance or power. Gas engines must maintain a proper air/fuel mixure ratio otherwise you get a slew of negative issues, some of which can destroy the engine (pinging/predetonation, etc). This is also why they sell three grades of gasoline while there is one grade of diesel. Octane is the measurement of the speed at which fuel burns. And finally, this is why you can turbo charge diesel engines fairly easily and even with turbos that were not ever meant for that vehicle, for example a Toyota 3B diesel with a Mitsubishi small gasoline car turbocharger attached to it.

Previously I lived at low level altitude back east. My NA ("naturally aspirated" aka non-turbocharged or supercharged) HJ60 with a factory Toyota non-turbo diesel was about 25% power loss at altitude difference between here and back east.

Back east my HJ60 was noticably more powerful than even my dad's V8 powered pickup truck. It went up the hills faster and I even towed a FJ40 that I sold, on my trip out here, to a gentleman in Chicago.

When I encountered the trip up through Kansas, going west up to the Mile High City, I started noticing black smoke in the tail pipe and loss of power. By the time I got to Colorado it was pretty bad. I actually thought something was genuinely wrong with it, and when I got to Utah I had the injectors rebuilt which did nothing other than increase smoke with no gain in power (because hte new injectors were delivering more fuel). It turned out to be nothing other than flat out altitude.

NOW, some engines have altitude compensators. All these do, similar to adjusting the fuel mixure screw on a carburetor, is reduce the amount of fuel proportionally to the detected altitude. So there is still a low of power with altitude on all NA engines. The percentage of power loss of a NA diesels is about 3% for each 1,000 feet. So at 3,000 feet it is about a 10% power loss. Where I live it is about a 20-25% power loss. I don't know what the percentage is for gasoline engines, but in general they take in less air on average so they are not as severly affected.

Ideally, the best high altitude engine is turbocharged, period. Because of the balance ratio needed for gasoline engines (because compressed air is still affected) a turbo diesel would be better than a turbocharged gasoline. Second best would be a turbo gase engine, or a noticably overpowered gas engine like a powerful V8 that you will not notice the power loss as far as performance. A NA gas engine with an altitude compensator would be best then there is no rotten-egg too much gasoline smell, common to what you smell from rockcrawlers in Moab. Finally followed by regular NA diesel engine which would absolutely have the worst performance in alitude, unless it was also drastically overpowered and also had an altitude compensator to reduce smoke. Altitude compensators with diesel are not necessary, many can simply be turned down with the turn of a screw. So if you are at elevation for weeks or months without dropping or raising noticably, you simply adjust the fuel ratio.

 

[R_Lefebvre]

The difference between gasoline and diesel engines is at the root of their thermodynamic cycles. They operate very differently. That is why they have different fueling requirements.

Diesel engines do not have a set A/F ratio. Basically the move as much air as possible, the intake tract is always wide open. The throttle pedal actually controls how much fuel is going in. They can run as low as 100:1 Air-Fuel-Ratio (I think, roughly) probably up to about 12:1 AFR. Somewhere along the line is the "smoke limit", the point where smoke is visible from the exhaust due to there not being enough oxygen content to fully combust all the fuel.

When you went up in altitude, the air density was reduced, which changed the effective AFR for a given throttle position. They would normally tune the system to operate below the smoke limit at sea level. At altitude, full throttle would get you over the smoke limit.

Gasoline engines typically run at a fixed AFR of 14.7:1. That is their optimum setting for emissions. The gas pedal controls the airflow, and the fuel flow is fixed to the airflow by the fuel injection system. You control the airflow with your foot (via the throttle plate) and the computer measure the air mass, and injects the appropriate amount of fuel. Fuel injected engines should not suffer from altitude sickness because of this. They just make less power because of the reduced air mass flow.

The problem you talk about with Jeeps in Moab, would be because of carburetors that need to be retuned for altitude, or really cheap and/or old fuel injection system that can't adapt themselves.

Just to introduce myself, I am an ex-Ford powertrain engineer and have training in engine calibration. So, ask away.