Diesel Pusher upgrading to heavier toad-advice needed

Interesting. Do we have any thermodynamics trained individuals out there who might comment on this. When I went through my engineering thermo courses it seems to me that the overall power (efficiency) of ANY heat engine (gas, diesel, turbine, etc.) was always higher as the temperature spread between engine operating temp and outside air temp increased. In plane language higher efficiency as it got colder out there.

As far as the airplane example was concerned, the reduction in payload was caused by a reduction in density altitude at takeoff. That could have effected the airplane two ways. Either the runway was too short and the reduced engine efficiency did not allow a the heavy plane to accelerate to takeoff speed before you ran out of pavement, or the takeoff speed needed to be higher due to less dense air caused by the heat and airport altitude, and you were still in danger of running out of pavement. Each aircraft has performance diagrams that tell the pilots (or commercial airlines dispatcher) how the aircraft is going to perform with a given load, temperature, and density altitude.

JBouknight wrote:


I don't know about everyone else but after reading that, I am going to have all of these calculations going on in my head while driving down the road.LOL The newest of which is the cold air temp robbing my MPGs. :E
Thanks for the interesting info!

Aerodynamic drag changing with temperature should come as a big surprise. More molecules to "hit" at colder temperatures.

I KNOW pilots know this. Heck, this summer we just flew non-stop Dallas to Sidney, Australia. Airbus 380. Sold out flight. 1/4 seats EMPTY. With hot temperature in Dallas, not enough lift (not enough air molecules passing over/under the wings) with full plane (ya and enough fuel for the 17+ hour flight).

Our MPG in summer running the dash air is still better than freezing temperatures trips without the compressor running.

wolfe10 wrote:
Bottom line (based on physics):

Extra weight has little effect on flat ground once up to speed.

Extra weight makes a difference when accelerating.

Extra weight makes a more significant difference when climbing grades.

Go to this link and download the PDF. Written by Caterpillar Corp, but same physics applies to all vehicles.

***Link Removed***

I don't know about everyone else but after reading that, I am going to have all of these calculations going on in my head while driving down the road.LOL The newest of which is the cold air temp robbing my MPGs. :E
Thanks for the interesting info!

We burn gas, not diesel, but we recently switched from towing an 1800 lb smart car to a 4400 lb Jeep Wrangler. Out average fuel mileage went from just under 7 mpg to just over 6 mpg...

wolfe10 wrote:
Bottom line (based on physics):/snip/



http://forum.dieselrvclub.org/index.php/topic,6594.0.html

Interesting read, Brett, thanks.....Dennis

We tow a 4800 lb Jeep, a 5500 lb Sierra or the Sierra with a golf cart and lots of other stuff at around 7000 lb; there is no big difference.

Bottom line (based on physics):

Extra weight has little effect on flat ground once up to speed.

Extra weight makes a difference when accelerating.

Extra weight makes a more significant difference when climbing grades.

Go to this link and download the PDF. Written by Caterpillar Corp, but same physics applies to all vehicles.

http://forum.dieselrvclub.org/index.php/topic,6594.0.html

Great information! Thank everyone for your replies.

Upgrading from our 3,200 lb CRV to the 5,450 Jeep we could detect the additional drag during acceleration. And grades a little more so.

Our fuel consumption went from 8.0 to 7.8. Very difficult to determine if this change was from the increased weight and/or other variables. 60-62 MPH 400 ISL

you will see the difference on the hills. No matter if its in the slip stream, and has little friction you still have to "lift" it to go up hill. Still shouldn't make much over all difference.