Why are tractor-trailer rig so much more fuel efficient

[jamesqf]

Originally Posted by trebuchet03

Trolling services for this bridge are no longer necessary

I suppose your next step is to claim that Hitler said diesel/electric locomotives weren't hybrids :-)

[TKWPrime]

All of you are close to the answer. Part of the FE of a big rig is the frontal area to mass ratio. That part was dead on. The other item to consider is gearing. When you look at horsepower to weight, a typical semi has fewer horses per pound than a car but with 18 to 24 sppeds, the gearing lets them run the engine near its optimum specific fuel consumption rpm at any speed. Once they are up to speed and rolling, the engine is running near optimum rpm and very highly loaded for the engine size.

The penalty for that is, naturally, very poor accelleration. 0-60 in 25 seconds is pretty slow by comparison to any car.

[Unforgiven]

As a current semi driver I can offer a little insight here. Tractor trailers are becoming more and more fuel efficient in tiny leaps and bounds. But weight and road conditions are key factors in how well it does on its run. Main factor, is gearing matchup with engine rpm, and finally rear axle ratios. The common transmission is the Eaton 10 speed, with 13, 15, 18, and even a few 21 speed setups available. Add in the new automatics (turns head to not puke) and the possibilities on gearing are many. Add in the rear axle ratio which can run from 3.55 to 5.23 (other ratios do exist but not certain on the numbers) and again, the engine is allowed to run more efficiently in its powerband. Idle is normally between 500rpm to 700rpm, with normal operating rpm between 1400 to 1900rpm. The Freightliner Columbia that I run in, typically gets 6.5 to 7.2 mpg, with 4.8 being the low so far, and a high of 7.7mpg. Add in the twin 100 gallon tanks (actually only hold 90 gallons each due to cap angle, and internals) and the range can be pretty good. Now another made the comment why cannot cars get 80mpg if the tractor-trailer can get 5-7 mpg. Well, dig hard enough and there have been vehicles that have gotten that kind of mpg. Sadly, the automotive gurus and makers decided that economy was not a selling point, but that speed and horsepower is. So, when Mopar actually had thier 2.2L four cylinder actually break 90mpg they hid it and the specs for it. GM had a 5.0L V-8 back in 198x break 40mpg and guess what... they hid it as well. Recently some inventor in Iowa (if memory serves, cant find the article at the moment) made their vehicle mods to allow it to run well above 50 mpg. It is not a matter of why cant a car do it, it is more a matter of why does big business wish to hide and deride the engines and vehicles that can.

[jamesqf]

Originally Posted by Unforgiven

...why cannot cars get 80mpg if the tractor-trailer can get 5-7 mpg.

Mine does :-) On a good day, anyway.

[trebuchet03]

It is not a matter of why cant a car do it, it is more a matter of why does big business wish to hide and deride the engines and vehicles that can.

Not so much big business, it's a matter of emissions. Sure, we want our oil to last, but we also need to breathe.

It's fairly easy to significantly increase fuel economy if you throw out emissions

What sucks is that we're already fairly close to the max thermal efficiency for otto engines given operating temps (can't really increase without screwing emissions) and compression ratios - there's not much room for growth. Of course, tech changes all the time and both current and theoretical max efficiency goes up - but it's a slow process. Personally, I believe that money is better spent in energy storage devices (batteries, caps, etc.) - where an otto cycle in a car might be around ~25% max theoretical efficiency --- steam turbines can have efficiencies much higher than 50% (which is why a majority of the world's power is driven by steam). Just my .02 cents

For everyone's reading pleasure
Fuel saving - a professional engineer's view

[RH77]

Originally Posted by jamesqf

Mine does :-) On a good day, anyway.

Where's your garage entry, BTW???

RH77

[Unforgiven]

Heh, remember the Chrysler Turbine of the late 60's? Would be interesting to see some amalagation of the Wankle and a turbine... some interesting half-baked ideas in my brain... lol

Emissions have long been a centerpoint of arguement and controversey. I do like breathing, although some might want me to stop. As far as the energy storage, I tend to prefer the mix of storage and micro engine power supply. I feel that hybrids rock, and we need more of the diesel-hybrid setups to find the way into the mix. Given the torque effective in a diesel, added to the electric motor torque, and then factor in power going to recharge, I am thinking that we could get diesel motors of 1.5L and a pair of electric motors powering vehicles like full sized vans and still getting great economy. Granted there is a lot more for me to study up on, but I dont think I am too far off the mark here.

[pugmanic]

Engine size to wehicle weight ratio. (smaller engine under higher stress is more economical)

Time at cruising speed.

Alsorts of reasons...

[solarguy]

Many good points already mentioned. I will add two minor ones.

1. Yes, diesel fuel contains more BTU per gallon, but a diesel engine with substantially higher compression is inherently more efficient than a gasser, all other things being equal. This is a direct consequence of Carnot efficiency. The stripped down version of Carnot efficiency says the higher the delta T (that is combustion temperature minus exhaust temperature) the more efficient, period. Higher compression produces higher combustion temps, all other things being equal. Less pumping losses was mentioned and is true also.

2. A semi has less surface area than a car, per pound. Quite a lot less. So even though they look as aerodynamic as a brick, there is less surface area to produce drag.


Finest regards,

troy

[jamesqf]

Originally Posted by solarguy

A semi has less surface area than a car, per pound.

I think you mean cross-sectional area? It's something people tend to forget when they talk aerodynamics. Cd is a function of shape, so a low Cd is better, but drag is Cd * cross-section. So even though the semi may have a high Cd, the cross-section relative to total weight is much smaller, so drag per pound is less.