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Old 05-06-2011, 10:02 AM   #1 (permalink)
10MPG w/ 8.5'x16' Trailer
Join Date: Dec 2010
Location: Morehead City, NC
Posts: 11
Thanks: 1
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Project Aero Car Trailer

I've become fasinated with the idea of making a super-efficient tow rig after reading some of the great info on this site and others. Y'all have some great ideas, backed up sound research and testing. That's so refreshing to see...

A little background on me: I'm a Mechanical Engineer working for Naval Air Systems Command (I rebuild helicopters). I also race Spec Miata in the southeast, at tracks like Virginia International Raceway (VIR), Carolina Motorsports Park (CMP), Roebling Road Raceway (RRR), Rockingham Speedway, Road Atlanta, and Summit Point. My wife is a budget analyst for the Navy, so she controls my money, both at work and at home! She's also very good at cost/ benefit analyses and things of that nature.

The tow rig: 2004 Chevy 2500HD with a 6.0L gas engine, 4-spd auto, and 4.10 gears. 108k miles, completely stock, and a very competent machine. I've been happy with it thus far. It get 14mpg unloaded at 60mph (2200 rpm), so I pretty much use it only for towing or hauling. I have a tonneau cover that I never use, because it prevents me from putting the ATV or motorcycle in the bed. I actually have it up for sale, so I count its use out. Otherwise, I ride a motorcycle.

The trailer: 24 ft box enclosed (box) trailer. Radiused vertical leading edges, and that's about it. It at least has drop-axels, so it rides lower than most, with about 8 inches of clearance. This isn't always the best, because it scrapes frequently coming into/ out of gas stations. And, as you can imagine, I stop for gas a LOT. Trailer estimated weight is 7,000 lb: 4300lb empty, 2300 lb car, 400 lb tools/ gear.

Current performance numbers: ranging b/w 8.5-10.5mpg (~9.5 avg.) towing at 60mph. Going faster would be really nice, since I live a minimum of 4 1/2 hours away from any track, but it sucks fuel so quickly I can't justify it. I also feel as if I have good power for what I need. Climbing hills is now problem; I could maintain 60mph up a decent hill if I really wanted to. I usually just stick it to about 1/2 to 2/3 throttle and let it slow down as it wants to. I don't feel like I need more power, which is something to say if you're a racecar driver/ sportbike rider!

The project: I think some huge improvements could be made to my setup! If an 18-wheeler, loaded with 80,000 lb can get 7mpg at 60, I'm sure I can do a whole lot better!

1) Streamline the transition between truck and trailer
-add a bed cap/ camper shell, hopefully one that angles up at the rear
-possibly add wings/ fairings to the bed cap to further direct airflow
-add a contoured trailer nose to guide the airflow from the truck to the sides and top of the trailer.

2) Reduce the turbulent wake behind the trailer
-a tail section that is able to be removed/ moved easily to allow ingress/ egress of the car seems like it would be very helpful. I believe what I'm envisioning is called a Kamm back. I'll post a link to a commercial product I've seen when I find it.

3) Operate the truck as close as possible to its highest efficiency range.
-difficult to determine, since I can't seem to find a BSFC chart for anything close to my engine.
-Judging by the relatively high throttle use required, the somewhat-high RPM, and the relatively small drop in mpg I'm guessing I'm pretty close as-is.

4) Change gearing as is appropriate after reducing aerodynamic drag.
-Most likely a 3.73 or 3.55 gear. This would obviously help non-towing fuel economy as well.

5) Use a more efficient drive train (most likely not happening until the current units fail).
-swap to a diesel. The OEM diesel is a 6.6L unit with WAY more power than I need. I think it'd be fun to install something not intended for this truck, like a cummins or mercedes diesel. :-) Suggestions on specific engines and where to get them?
-manual transmission. Oh, how I'd love this. Complete control, more gears, more efficient. I'd like to do this with any engine. If I could have found my truck with a manual, I would have!
-convert to E85. I'm not convinced this is a good idea, but it sure would be cool. Since E85 is cheaper, and has a higher octane rating, it might work. I've tossed around the ideas of increasing the compression ratio of the engine, and would definetly need to add a turbo. I'd also need to upgrade the fuel system to handle the extra flow rate required, and reprogram the electronics to expect the correct fuel. Also, I'd have to bring a large (100 gal?) auxiliary fuel tank, as E85 stations are few and far between. Luckily, there's one a few miles from my house, but there isn't another one for 100 miles or so. Running E85, I could potentially have more power on tap (turbocharging), plus run the engine at a more efficient zone by requiring more airflow for the same power output, and getting that airflow by turbocharged/ intercooled induction. I've heard that starting an E85 engine in the cold can be difficult as well. This one's a big investment and a big risk, with a lot of factors involved!

So why post this thread? I'd like to get y'all's comments and suggestions on improvements to be made. Keep in mind the truck has a large cooling demand, both for the brakes, and the engine/ tranny. The whole point of improvements is to save money in the long run, so without a business analysis, no projects are going to go anywhere! The queen says NO!

I need the most help finding an accurate method for estimating the fuel savings to be gained by making these modifications, so that a return-on-investment calculation can be run.

I also want to be able to share with other towing folks what I do and if it works. I searched and didn't find anyone else describing their experiences towing a large trailer, but they're out there!

Soon to come: sketches of my rough ideas for fairings and aero improvements.

-Matt McBride

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