De-rate for altitude?

[Gary RV_Wizard]

Traditional wisdom is that the performance of a "normally aspirated" internal combustion engine decreases about 3% for every thousand feet of altitude, because of the decrease in the density of the air.  That was a good number in the past, but I believe it no longer applies to any modern vehicle engine. My  reasoning is that no modern engine (in the USA anyway) is "normally aspirated" within the meaning of this venerable rule of thumb.  First, of course, is the fact that many modern engines have turbo chargers or even superchargers and are not in any sense "normally aspiraed".  Turbo charged engines get all the air they need rammed into them by the turbocharger.  But second, even non-turbocharged modern engines still have their air/fuel mixture managed quite ridgidly by an engine control computer that measures the density if the incoming air charge and adjusts the air/fuel mix accodingly. To meet fuel economy and emissions goals, the air/fuel mix is always kept at it stoichiometric value, which is 14.7:1 (air-to-fuel) for gasoline. This ratio is maintained at any altitude and any temperature within the vehicle's designed operating range.  If the density of the air decreases, the control system increases the amount of air in the air/fuel charge until there is sufficient oxygen for a proper fuel burn, i.e. a 14.:1 ratio. Even if the control system misreads the air density, feedback from the oxygen sensor in the exhaust will tell it if the mix was too lean or too rich and it will further adjust until the fuel is burned fully, yielding optimum power as well as minimal pollutants.

I don't know just how much altitude the various engine designers planned for (or that the EPA requires), but you can bet that it is at least 10,000 feet because many regions in the USA are 5000 feet and above.  I just spent several days in Montana and Wyoming without ever coming below 6500 feet.

Given this, in my opinion there is no longer any reason to allow for any decreased engine performance at the sort of  altitudes an RVer may encounter.

Comments, anyone?  ???

 

[Tom]

That sure makes sense to me Gary. I wonder if anyone can confirm it by having driven a current model vehicle from near sea level to high altitudes (can be done in a matter of hours in Northern CA).

On the flip side of today's engines, I recall Chris owning a Triumph Spitfire in the early 80's. This little rag top with a 1200cc engine was a nippy vehicle running around the relatively flat Bay area. But, when we took it over Tioga Pass (9,945 feet), I wanted to get out and walk.

 

[Carl L]

...To meet fuel economy and emissions goals, the air/fuel mix is always kept at it stoichiometric value, which is 14.7:1 (air-to-fuel) for gasoline. This ratio is maintained at any altitude and any temperature within the vehicle's designed operating range.  If the density of the air decreases, the control system increases the amount of air in the air/fuel charge until there is sufficient oxygen for a proper fuel burn, i.e. a 14.:1 ratio. Even if the control system misreads the air density, feedback from the oxygen sensor in the exhaust will tell it if the mix was too lean or too rich and it will further adjust until the fuel is burned fully, yielding optimum power as well as minimal pollutants....
....

Comments, anyone? 

Yeah.  I agree with you on on turbo- and super-charged engines.  However, on a normally aspirated engine at high altitude, the intake stroke simply draws in less air than at sea level.  In dear old days of carburation, unless a rig had a high altitude tune on the carb, you laid down exhaust like a coal burner from an over-rich mixture and lost power like speed was going out of style. 

Nowadays with computer controlled fuel injection, you get that high altitude tune automatically.  Again however, without a blower to pack it in, the air is still going to be thin, with a lower partial pressure of oxygen.  Therefore, you computer is going to lower the fuel charge.  You wiil then have less fuel to burn, just as if you had throttled back.  Less fuel, less energy produced per explosion, the less horsepower developed -- the reduction number I have seen is the 3% per thousand foot altitude.

I know that turbocharging is common on diesels.  Supercharging is used on highly tuned cars, race cars, and small cars to gain the equivalent HP that would normally.  However, I have not seen either on truck gas engines.  For that matter I would ask, is it universal on diesel truck engines?

 

[Karl]

Sorry Gary, but I have to agree with Carl on this one for the same reasons. The mass air flow sensor combined with the oxygen sensor are the major contributers to the fuel/air ratio adjustment. Less dense air; less gas provided.

 

[Jeff]

Gary:

On computer fuel injected aircraft engines the max manifold pressure drops from approximately 27-28 inches of mercury at sea level  to about 18-20" at 10,000'.  That represents a substantial (40%) drop in available power).

For aircraft the parasite drag of heavier air also decreases so airspeeds do not drop that much but unfortunately parsite drag is a small portion of the work in a motorhome.

 

[Just Lou]

While replacing my air filter yesterday (on a EFI Ford 460) I was following along in the manual to familiarize myself with the terminology
used to define all the hoses and parts I was removing.
 
Two of the smaller hoses that I removed from the input air scoop were described as coming from the secondary pulsed air injection pump.  I'm not sure if this air is constant or regulated.  Could it be in place to limit the effect of thinner air (maybe restricted air flow, as in dirty filter)  Is it relevent to this discussion?

 

[Gary RV_Wizard]

The engine computer is metering both fuel and air. Sure, the piston can suck only so much air, but the amount of air available is usually much more than is needed for current power demands. If the ECM always provided enough fuel to hit the 14.7:1 ratio for a cylinder full of air, the vehicle would have only one speed: wide open throttle.  Instead, the whole system is constrained by the accelerator  pedal.  When power demand is low, fuel consumption is cut back but the engine controller still maintains the proper 14.7:1 air/fuel ratio.  So I would have to agree that the maximum amount of power available may be reduced somewhat, but in the general case there is sufficient air available in the incoming air charge to properly burn the fuel needed to produce the power the driver demands via the accelerator pedal. At many (most?) levels of fuel consumption,  the engine is not starving for air as a fixed-tune carbureted engine would be and the engine (and thus the vehicle) performs normally.  Wide open throttle performance is no doubt less, but how often do you have the pedal all the way to the floor?  I just spent several days in both car and motorhome at 4000-8000 feet altitude and don't think I ever once had the accelerator to the floor, even when climbing steep grades. Both vehicles are gas powered and neither is turbo-charged.

 

[Karl]

Good point Gary and absolutely correct, but there have been many times ascending steep grades that I've had my foot to the floor and wished there were more pedal ;D

 

[Gary RV_Wizard]

but there have been many times ascending steep grades that I've had my foot to the floor and wished there were more pedal

For the sake of completeness, Karl, what vintage engine are we talking about here? Is this in your Bounder?  Engine controls have improved steadily over the last 10 years and recent engines do better than older ones, even though the older ones also have active combustion management systems. And newer vehicles may have more capable multi-speed transmissions, which helps quite a bit too.

 

[blueblood]

I don't know just how much altitude the various engine designers planned for (or that the EPA requires), but you can bet that it is at least 10,000 feet because many regions in the USA are 5000 feet and above.? I just spent several days in Montana and Wyoming without ever coming below 6500 feet.

Given this, in my opinion there is no longer any reason to allow for any decreased engine performance at the sort of? altitudes an RVer may encounter.

Comments, anyone?? ???

I've had been running my new Cummins ISL400 at from 10,250 to 11,000 in CO for several weeks and it performed flawlessly. It of course does have a turbo as has every diesel I've seen in last twenty years or so. I also rode a fully loaded tour bus with a Cummins at 10,250 and it climbed mountain nice and smooth at legal speed limits. However, I would add that all Cummins IS series engines except ISB275 have a barometic pressure sensor in the engine at for high altitude to prevent damage from overfueling. 

 

[Carl L]

Wide open throttle performance is no doubt less, but how often do you have the pedal all the way to the floor?  I just spent several days in both car and motorhome at 4000-8000 feet altitude and don't think I ever once had the accelerator to the floor, even when climbing steep grades. Both vehicles are gas powered and neither is turbo-charged.

Cannot say that I have not had the pedal to the floor with my little '95 Ford shortblock 5.0L but then I down shift and that cures the problem.  And, yes, I do a lot of traveling at altitude in 2nd and 1st on grades.  The computer keeps up nicely with the mixtures, but as I said, there is less fuel to burn and less energy produceed per unit time.

SAE horsepower is evaluated on a standard test bed under strict standards.  It is observed, and witnessed performance -- not calculated.  As as I can figure out there is no evaluation of altitude effects and the test is run at normal atmospheric air pressures.

Do we have a member of the SAE around here?

 

[Ron]

Also horse power ratings are given for sea level.

 

[Karl]

Gary,

Yes, it's the '96 Bounder with the Ford 460 and throttle-body efi, and the E4OD tranny; not the greatest in terms of seamless shifting. The new exhaust system gave quite a performance increase and the Banks TransCommand really helped firm up the shifts. I will do something to increase the breathing`also, but haven't decided what yet. It will be passive however; no turbo or supercharger.

 

[Gary RV_Wizard]

Do we have a member of the SAE around here?

As a matter of fact, I used to be (as part of my professional life) but no longer keep it up since I retired.

AS Ron says, SAE HP ratings are measured at sea level.  There are actually a couple different SAE HP measurement standards, gross, net and the most recent certified. Most current engines have SAE net HP ratings, but SAE Certified ratings are becoming more common now (started in 2005).All measure power at the flywheel, not the drive whees, so do not take transmissions and drive trail losses into account.

For the SAE J1349 relative horsepower calculations, the standard reference conditions are:  Air temp 77 deg F (25 deg C),  29.235 Inches- Hg (990 mb) actual pressure and 0% relative humidity.

 

[Gary RV_Wizard]

Following up on the question of available air, consider that on each intake stroke, each cylinder of an engine such as the 8.1L GM gas V8 takes in about 62 cubic inches of air!  It's actually a bit less than that because not all exhaust gasses are vacated in time  to be replaced by the incoming air charge, but even allowing for a 20% decrease due to exhaust there is around 50 cubic inches of air available to support combustion. That air contains less oxygen at 5000 feet than at sea level, but it still has a lot of available oxygen for combustion.  The amount of fuel injected per cycle in each cylinder is infinitesimal, far less than even 0.1 cubic inch, so not all that much air is needed.  The ECM measures the absolute density of that air, which corrects for both temperatre and altitude, and adjusts the air/fuel mix appropriately.  With sufficient air on hand, the ECM is able to largely compensate for changes in temperature and altitude, even though the air density is less.  I will grant, however, that the correction is not  100% effective across the full RPM range and there is ultimately some loss of max HP.

Some engines now have active valve control, e..g variable valve timing ala Honda's VTEC, but I don't know of any in common RV engines or major tow vehicles, so I'll leave them out of the discussion. Active valve control mechanisms give the ECM some control over the amount of the air charge as well as the amount of fuel and provide a further method of on-the-fly tuning.

 

[rbell]

Very interesting. I've dealt with altitude on several different types of engines so I just had to read this. It brought back some good memories.
My first trip snowmobiling in the mountains was in a 6 cyl Ford van and a trailer about 2500 lbs. Going over Togowatee pass about 9500 ft. it got the grunts so bad I wasn't sure I'd make it. The snowmobiles had to have the jets changed more than once also.
Another trip with my son was in a Bronco 302 FI? and it did pretty well. The snowmobiles had HAC on them. It was a High Altitude Compensation device that changed the float bowl pressure in the carbs. This leaned the mixture as the altitude went up with a sealed chamber device and a tapered needle. A later model I had controlled the float bowl pressure electronically with barometric and temp sensors. It worked pretty good till about 10,000 ft. After that I started to run low on air, a bit more rest was in order when you got stuck. I had another one with direct injection between 11k & 12K ft. It ran very well but the HP gets pretty low that high. The 3% rule is for a well tuned engine but the air is thin up there. You can really tell when your digging out. The best tow vehicle we ever used was a Ford PSD and a 7000 lb trailer. It just didn't know we went up.
In my foolish youth I flew several airplanes in the mountains of BC, the Yukon, & Alaska. The ones with fuel injection did pretty well till about 10k ft. They were manually adjusted using exhaust gas temp. After that I'd feel the effects of altitude, it's pretty subtle. One was a float plane with manual fuel injection and dual turbos, one on each side of the engine. It would hold sea level manifold pressure forever, but I never had it over 15k ft. I didn't have oxygen and the air is really thin up there. You could also over boost it and get off the water in some pretty tight spots in a hurry, with just a little too much load!!!!!
Now my biggest problem is getting my Expedition & trailer over the mountains in Kentucky & Tenn. They just seem like big bumps when I look back on things.
BTW That picture I just added is me sitting at about 10k ft with the Tetons in the background.

 

[Gary RV_Wizard]

Thanks for the report of actual performance (vs theory).  I know my 99 Ford PSD w/turbocharger didn't seem to notice altitude, though I never had it above 9,500 feet.  Iv'e had my current gas engine in the motorhome (8.1L, no turbo) up around 8600 feet several times and it performs excellently and does not act short on power. However, I noticed the cooling fans were running a lot even though temps were in the 40's, so I think it actually was working harder than it does at 2000-3000 feet. Apparently it was able to compensate OK.

 

[Lou Schneider]

I think what's being missed is the absolute air pressure.  At sea level the absolute air pressure is about 30 inches of mercury (29.96 inhg).  The same pressure at 5000 feet is 24.90 inches, at 10,000 ft. it's 20.58 inches.

It seems to me that as long as the fuel-air ratio remains constant (does not go overly rich), a gasoline engine that is running 10 inches of intake vacuum at sea level will produce the same amount of power as it will under full throttle at 10,000 ft. elevation.  In either case the piston is pulling air at an absolute pressure of 20 in hg. 

Turbocharged engines will fall off by the same amount - less absolute pressure into the charger = less absolute pressure out, unless the turbocharger is hitting the wastegate cutout pressure at low elevations.  It could be that the Powerstroke turbocharger is large enough to provide sufficient air at altitude. 

Maybe the missing factor is wind resistance at altitude?  The majority of a motorhome's power is used to overcome wind resistance, and I'd think the thinner air at altitude would generate less wind resistance than the thicker air at sea level.  Perhaps this is dropping off at roughly the same rate as the engine's horsepower at altitude, giving the impression that the engine is still producing full power?

 

[Karl]

The point that everyone seems to be missing is that all other things being equal, a larger cubic inch engine will produce more full-throttle horsepower than a smaller engine at full throttle - more fuel/air mixture, more horsepower. At full throttle, something that's sometimes required when climbing grades at higher altitudes, the engine acts like it were smaller and produces less horsepower. Without turbo or supercharging, it will still draw in the same number of c.i of air and mix it appropriately with the right amount of gasoline, but the charge being less dense, it can't produce the same power. We seem to be trying to compare the part-throttle fuel/air quantity necessary to maintain a given horsepower output with net available power at full throttle - apples and oranges.

 

[Karl]

Lou,

Turbocharged engines will fall off by the same amount - less absolute pressure into the charger = less absolute pressure out, unless the turbocharger is hitting the wastegate cutout pressure at low elevations.  It could be that the Powerstroke turbocharger is large enough to provide sufficient air at altitude.

A turbocharger doesn't need to be at the pop-off pressure to benefit at lower altitudes. As long as it is providing positive pressure to the intake runners, it is putting a greater fuel /air charge into the cylinders and produces more power. The wastegate functions solely to relieve pressure upon throttle closure and to prevent over-boosting which could cause piston or head damage.