Turbodiesels and Altitude/Temperatures

[Shannow]

Going on from the NA/FA thread a week back, had an interesting experience today.

Driving home from holidays in 40-43 degree temperatures, from sea level up to 1,000m

I've always noticed that the drive from sea level up to here involves some, albeit small lack of performance, today I repeated the trip today in 40+C temperatures.

The vehicle (3 litre turbodiesel, direct injected) felt seriously a half a gear out on any sort of climb, roll on response , overtaking manoeuver.

Trip time was a little slower than usual (not much), but I seemed to be rowing the vehicle in the heat.

Temperature gauge stayed absolutely normal fuel consumption on average, just seemed to be a power issue.

 

[1sttruck]

Air density is less, power drops a bit.

 

[Drew99GT]

That's over 100 degrees F, so that also contributes to less are density. Plus, the intercooler becomes less affective and it's a downward spiral. I'd imagine the corrected density altitude was over 2,000 meters; maybe even more.

Here in CO (where we measure things in feet!), with an altitude of 6,000 feet at the airport, on very hot summer days, we can get density altitudes of over 10,000 feet. Meaning the airpressure and density would be the same as being at 10,000 feet.

You should try flying a Cessna in those conditions. When I was taking flight lessons my instructor took me up just to feel it. We barely cleared the powerlines at the end of the runway and the oil temp instantly went into the red on climb out so he basically did one trip around the pattern and idled back to the runway to get the engine cooled off.

 

[Shannow]

Intercooler isn't part of the equation. The ute didn't come intercooled.

Yep it was a density issue for sure.

The spark ignited cars in town were almost all knocking as they were moving off.

 

[Shannow]

speaking of altitude, 5 days at sea level and you pose the respect for the sun that you need at 850m.

Didn't get burned at all down there, days in the sun...a day at home.

 

[JTK]

Could you imagine that hike with a NA diesel? You'd have to hop out and help it along... Putt...putt..click..clack..SMOKE!

Joel

 

[Shannow]

I did that when I had a 1987 BJ42 Landcruiser.

Went down the Abercrombie, and up the other side each way.

I thought that I was one of Aeneas' rowers

 

[mechtech2]

You would expect to give more throttle at intermediate settings, but the turbo should build pressure to the same level. It has a a wastegate to control the max pressure.
It may take longer to spool and get up there, but the max should be the same.
Possible the warmer temps affected a sensor, and the engine had a fuel curve change because of it - thus less power.

 

[Drew99GT]

Originally Posted By: mechtech2
but the max should be the same.


Even if you get the same boost pressure at high altitude, there's less "air" if you will even with that same boost (less density/less oxygen molecules). Plus, in order to attain that same boost pressure, the turbo has to spin faster, which lowers efficiency and heats up the intake air even more.

Factory turbo cars at our elevation of 6,000 feet still run roughly a half second slower then at sea level in the 1/4 mile, even if the wastegate can maintain the exact same boost level.

 

[JTK]

Originally Posted By: Drew99GT
Originally Posted By: mechtech2
but the max should be the same.


Even if you get the same boost pressure at high altitude, there's less "air" if you will even with that same boost (less density/less oxygen molecules)..


Exactly. If I remember correctly, your power in a diesel is dependent on how much air you can pack into the cylinders. The injectors are going to squirt in the same volume of fuel independent of air volume (on a MAF-less diesel I presume). There's no throttle.

Joel

 

[edhackett]

Subaru, for example, doesn't try and maintain the same boost level at altitude. They taper the boost with increasing altitude to limit turbo speed. It's done to aid long term reliability.

Ed

 

[widman]

In general the turbo makes up for the first 6000 feet or so of altitude. I just sold my 3.0 turbo diesel toyota, but it ran nicely up the mountains (this is my most frequent drive (in meters)

The drive from Sucre to Potosi (3000 meters to 4000 meters) was more than 2 hours with my 2.8 non turbo. The Turbo did it in 1.5 hours. The trick to that turbo in altitudes is keeping it above 2000 rpm. Let it drop to 1500 and their is little or no boost.
Next week I'll break-in my new 3.7 Grand Cherokee.

 

[Tornado Red]

Sometimes, for emissions-reduction purposes, high fuel temperatures will result in the engine computer retarding the injection timing.

With VW TDIs, some folks have found that "tricking" the engine computer into thinking the fuel is colder than it actually is, will result in better fuel economy. Not sure about performance, but this might be affected too.

Regarding intercoolers: yes, they are less effective in super-hot weather. But if you don't have one, performance is going to be degraded.

 

[Junior]

Yep, combination of things. At higher temperature, the fuel density is reduced as well, resulting in less power.