View Full Version : Why no fuel efficient haulers?
08-01-2006, 11:20 PM
I was just thinking, since the large percentage of energy required to drive comes from the coefficient drag and reference area, why not design a 8000lb aerodynamic statin wagon looking truck? As long as it has the mass and braking ability, it can still tow the same stuff. The difference would be that instead of getting ~20mpg@55mph, it'd could easily get twice that. Kinda like those huge station wagons from the 60s, except sleek and diesel powered. It could still have plenty of ground clearance too, the only differene would be that we're not pushing a wall down the freeway.
Why not design a 8000 lb aerodynamic station wagon looking truck? As long as it has the mass and braking ability, it can still tow the same stuff. The difference would be that instead of getting ~20mpg@55mph, it'd could easily get twice that.
___A 2006 Jetta TDI with an Auto receives an EPA of 35/42 City/Highway. Now add a heavy trailer to it and see if it can even make it up to highway speed let alone receive anything near 42 it was rated for while doing so? It just is not going to happen with a smallish 100 HP capable diesel :(
___I have read of some OTR’s that have touched 10 + in the big rigs as well as some of the 6.0 L PowerStroke guys touching 20 mpg’s while towing lighter loads cautiously however.
08-02-2006, 12:13 AM
Why would I use a TDI Jetta to tow something? The TDI's tiny... as is the Jetta.
What I'm saying is that there's no reason why a manufacturer couldn't use a nice sturdy truck frame with a cummins I6 and a body similar to the chrystler pacifica. As long as the gross vehicle weight, braking, and hauling are the same, it can do almost (if they had a "hide away bed", kinda like the ridgeline, then it could do) everything a truck can but gets better mileage.OTR rigs are designed based on the trailers, which are based off of the truck's ability to navigate roads w/o running over everything and it's mother. They could design a cab/trailer with half the CdA, but that limits cargo area too much, especially considering most cargo doesn't weigh that much. Passenger vehicles otoh don't have these constraints, but are still designed with lots of CdA for no apparent reason. To put this another way, at 55mph, a vehicle with a weight of 10,000lbs and a CdA of 10ft^2 compared to a vehicle with a weight of 3,000lbs and a CdA of 10ft^2 only uses 20-30% more energy even though it's three times the weight. Now when hauling large loads with gassers, building a more powerful motor means mroe pumping losses when that power isn't being utilized, but diesels don't suffer from this, so there's no reason why a powerful diesel station wagon that get's ~40mpg highway (probably better) can't be built.
___I was pointing out that the tiny Jetta TDI can just touch the FE you want while towing nothing. The new Volvo based (not the car company anymore) OTR’s with all the aero pack and trailers with aero side fairings, single tire wheels on the corners and a funnel rear faring are supposedly only going to reach 11 with a full load of Walmart supplies. Not bad but not that great either. If you are going to tow, you need HP. If you need HP, you are not going to reach 40 mpg by any means I can think of …
___Another consideration … The UW-Madison hybrid challenge vehicles. These were some of the best hybrid designed vehicles in the world. They were using smallish hybridized Diesels with lightweight uni-body and frame construction. These heavily modded SUV’s barely touched the mid to high 30’s let alone having the power to tow as well as reach even higher FE.
___A serial hybrid might be able to achieve the FE with the towing capability at slower speeds and acceleration rates but when you start talking about towing, you have to start thinking huge motors and packs to pull any kind of weight.
08-02-2006, 02:44 AM
The reason heavily modded SUVs barely hit the thirties is because they are SUVs, probably with CdAs of at ~12-15ft^2. The primary factor in highway fuel efficiency is CdA and engine efficiency. Since diesels don't suffer from reduced engine efficiency with reduced load, then the FE of a diesel vehicle on the highway is more or less based on CdA. I found some info on a 2005 Dodge Ram, supposedly it has a Cd=.45, FA=34.6ft^2, and CdA=15.6ft^2. Compare this to the Jetta with a Cd=.36ish, FA=21ft@ and a CdA=~7.6ft^2, and you can see why these chipped trucks only get ~20mpg compared to the ~42mpg of the Jetta. The ratio of CdAs is ~2.05, and the ratio of fuel efficiency at highway speeds is ~2.1. No penalty is seen fo weight because the truck's engine is being run leaner, and more efficienctly, but generally, there's a 5-10% penalty when adding a few thousand pounds of weight. If you can build Cummins diesel I6 powered station wagon that has the same weight/power/braking as a diesel truck then you'll probably see ~40mpg because for a diesel, highway mileage is primarily influenced by CdA. We are moving through a fluid after all... ;)
When you mention needing more hp to tow, the drop in efficiency only applies to gasoline engines. This is why a corvette has a CdA of ~6.5ft^2, compared to an Insight with one of ~5ft^2, and only gets about half the highway FE. When putting a ~400hp gasoline engine in the car, the manufacturer magifies pumping losses at highway cruising speeds, unlike the Insight, which has an engine that will run more efficiently and exhibit fewer pumping losses at highway speed. Diesels don't have this penalty for adding more power, engine efficiency is more or less related to the air/fuel ratio (swirl, injection pressure and pattern, can help too), so as long as I increase the amount of fuel and air linearly (with a turbo) then I can increase hp, and not have any change in efficiency, up to a point.
08-02-2006, 06:43 AM
Dodge Sprinter! :D
2.7L I-5 turbodiesel averaged 30 MPG in highway driving by the lead-footed Car and Driver journalists.
___I think we are talking past each other.
___To drive down the road at 55 mph only takes ~ 20 hp in a compact or small sedan. Look at the EPA test and then think Jetta TDI. There is a small aero drag factor inserted but the Jetta with its lowly HP can barely touch 40 mpg without anything else attached. Attach a trailer to it and its FE will sink fast no matter the aero as it has to carry the load up and down every overpass, small hill, long incline or mountain. The Jetta all on its own and having little in the way of HP available can barely hit 40. Beef it up to tow and then actually tow and your FE tanks. Aero drag is way over rated until you begin punching beyond 50 mph and the HP numbers at a 30, 40, 50, and 60 mph speeds prove it. It is when you have to spool the turbo and she starts sucking down fuel during any small climb to maintain speed where a small diesel’s FE tanks. The larger the Diesel, the more they suck fuel.
___WRT gas engines, their thermal efficiency improves mightily as you get into the higher throttle plate openings. That being said, they are running at such a low HP output at 55 they don’t have an issue until towing as well. The Insight’s ICE is barely any more efficient then any other. What gives it its highway FE is lean burn. The best thermal efficiency I have seen vs. bhp was 35% IIRC and that is not much as many diesels are in the 39 - 42% range.
___You need HP to tow up grades meaning larger diesels and when you use it, your FE tanks.
08-02-2006, 03:35 PM
20hp@55mph sounds like an SUV! A Jetta weighing ~1500kg, CdA=7.5ft^2~.96m^2, Crr=.015 at 55mph~21.5m/s only needs ~~23N to overcome rolling friction, and ~266N to overcome fluid friction. Since we're traveling at ~21.5m/s we're using 6.2kw~8.3hp, with the additional CdA and weight, a Dodge Ram may use near 20hp@55mph, but that's a big truck, and it uses that much because there's tons of energy used to overcome fluid friction.
In terms of comparing fluid and rolling friction, generally, at ~15-20mph they are roughly equal, so if you go faster than this you start having serious losses due to drag. So much so that around 25mph, your drag has increase by a factor of ~1.5, and it's roughly 3-4 times the energy required to overcome rolling friction.
Otoh, most drivers of gasoline powered vehicles won't see this because they suffer from increased pumping losses as they *drive slower, as the energy required to move at some speed decreases so does engine efficiency. You mentioned lean burn, which is a significant part (maybe 10-20%, I'm not sure how lean they can go) of the Insight's efficiency, but the fact that it has a ~70hp engine compared to the ~400hp engine on a vette means that the throttle plate's (or however airflow is restricted) open more, there is much less difference in pressure and much less negative work being done on the crank during the intake of air. In most cases, even though a gasser has ~35% thermal efficiency, the driver won't see this unless they're running wot and not allowing any negative work to be done by not letting in enough air. Diesels do not suffer from this, so if they cruise at lower speeds they'll see much better FE than gassers would if they drop the same speed. Of course Cd changes wrt speed, so this can vary, just like Crr, but the whole point is that for most vehicles (with the exception of OTR rigs) drag contributes much more than weight to the energy required to move the vehicle, and for diesels, it's pretty much directly related to FE. This is why I can go to the store with my friend in my Rabbit (~2500lbs) with a average speed of ~30mph when in motion, and all the idling,cold starts/8-12mile rt/2-4 stops, and get approximately the same milage I get during highway trips at a steady 55mph (~2200lbs). Even though braking/idling/cold starts waste a lot of energy, driving at ~55mph versus ~30mph uses about three times more energy due to fluid drag, and because I don't see pumping losses, I get about the same FE.
Yes, when towing in a beefy diesel station wagon, FE would decrease. But not primarily because of the weight, it decreases because the trailer isn't usually aerodynamic and increases the fluid friction substantiall compared to an aero wagon alone (not so much compared to a big truck). As for going uphill, of course FE will drop, but unless they're always towing something, and going some place new, up hill, that's not exactly pertinent to FE because they will eventually go down hill (using that extra PE as KE) and a round trip will have a change in elevation of ~0, the equivalent of driving the same distance on a completely flat piece of land. Anyway, the best argument for building aero diesel wagons that can tow, is that the majority of people who own these things don't use them for towing most of the time (~75%). If FE can jump from ~20mpg to 30-40mpg for most uses, then we just saved a lot of diesel, while still retaining functionality.
*Ironically, if I make a gasoline vehicle more aerodynamic, it reduces the energy (aka fuel and air) required to move at some speed, and pumping losses increase slightly. Obviously not as much, but it's still there, kinda like an M steps forward, N/M steps back, where M>N. ;)
Edit- Oh... The Sprinter with Mercedes Cdi engine (iirc?), cut the top 1/3rd off of that and I'm there! :D
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