“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, October 07, 2006


It's been almost two months since my last post due to some surgery that made writing and typing difficult. I hope I haven't lost my devoted readers. Right. In any case, onward and upward.

The trend in my mileage has been a significant increase in standard deviation, together with a slight decreasing trend in mileage. This, despite the installation of the K&N high flow air filter noted in my last post. Starting June 19, there was a major downtrend in my mileage from which I've never really recovered, followed by a trendless few tank fulls with large variation.

A comment was left in my post about Dr. Steven Dutch and his article about the 200 mile per gallon car. At the end of that post, I stated that it was my belief that "in order to achieve major reductions in oil consumption without going to vehicles such as the scooter I discussed a couple of posts back, large-scale changes must be made in the technology of internal combustion engines or other propulsion methods must be employed."

Bill Anderson, host of the blog entitled "mental radiation," commented that large gains can be made in automotive gas mileage by reducing the weight of vehicles. He stated that two thirds of the energy used at the wheels is used to overcome weight, and concluded that by reducing weight the amount of energy required to get from point A to point B can be reduced. Dr. Dutch implied a similar conclusion.

What about this? Well, obviously, since F=ma, that is, Force equals mass times acceleration, it takes more force to get a heavier (more massive) vehicle up to a given speed. But at speed, on level road, acceleration is zero and hence, the sum of forces acting on the vehicle must be zero. These forces are dissipative (drag, rolling friction, driveline friction, engine friction) and force applied to the road by the engine. With the likely exception of rolling friction, seemingly none of these are a function of mass, though engine friction must increase with engine size, which in turn typically increases with vehicle weight. Though this is probably not necessary by the laws of physics, a certain capacity for acceleration must be provided by its manufacturer to make the vehicle saleable.

And since I concluded in a series of earlier posts that engine friction is a very significant component of energy usage, heavier vehicles must use more fuel even in unaccelerated travel, though it isn't a direct correlation. Added to this, it takes more fuel energy to lift a heavier vehicle up a hill, energy which is not fully recovered in the descent due to disspative forces. Further, it seems very likely that heavier vehicles produce higher tire rolling resistance. In fact, this is almost certainly the largest contributor to increasing fuel consumption with increasing weight. Finally, unaccelerated travel on level roads for long periods is not the norm.

Thus, I agree that weight reduction is an effective means of increasing fuel economy, but at freeway speeds a large percentage of the force the engine must overcome is produced by drag. Reduction in "flat plate area" can be achieved by making cars smaller as well, but there is a limit - we still want a driver's seat and a passenger seat. I doubt we'll see tandem seating anytime soon.


Bill Anderson said...

Bill here. ;)

The efects of weight are generally larger than drag for the same reasons rockets are limited. A heavier vehicle requires more stopping power to provide reasonable stopping distances. This carries on to other aspects. A heavier vehicle requires more power to move. That means a heavier drivetrain from engine to axles and all in between. A heavier vehicle in turn needs "beefier" suspension components to manage/handle the forces involved in suspension travel over various surfaces at speed.

Add to this the lower fuel economy of increased weight requiring more fuel and thus weight and you can see how the situation is similar to rocketry, if not such a grand and obvious scale.

Case in point consider the early Corvettes. My 99 is much larger than the 63, for example, but significantly lighter. I get about 23MPG in town (no highway). The fuel injected smaller (327 vs 350) V8 in the smaller but heavier car achieved about 13 rated. Sure, aero is better yet my front plate is much, much larger. Changes in fuel management? Sure, but the modern Vette V8 is still a cam/pushrod settup. The main difference is in the weight. Placing a modern Vette V8 in the 63 chassis/body does little to improve mileage.

You are correct in there being a limitation to smaller sizes. Smaller size cars are inherently more dangerous to occupants than a similarly built yet larger vehicle. Crumple zones are the single largest contributor to increased occupant safety over the years. The more crumple, the more energy absorbed in the crash by the car and not the occupants.

Additionally, a narrower vehicle is less stable at speed compared to the same body in a wider version. Again, this is something race teams have learned, particularly Chevy with the C6 Corvette. They had to widen it back out to maintain stability at speed. A narrow vehicle will induce greater body roll and increase angles in the suspension requiring stronger components.

Length also has important stability and handling characteristics. A shorter wheelbase results in a more harsh and less predictable ride on non-level roads. This is the main reason a Jeep Wrangler feels bouncy compared to say a Cherokee with the same suspension components.

Anyway, just thought I'd leave some more food for thought on that issue. Cheers,

King of the Road said...

If what you mean by "the effects of weight" is the possibility of making relatively large reductions, I agree.

If you mean that the overall amount of fuel burned to overcome weight is higher than that used to overcome drag at highway speeds, I disagree. I calculated elsewhere that at 55 m.p.h., the Grand Cherokee must overcome about 383 Newtons of aerodynamic drag and 289 Newtons of rolling resistance.

The increased component weights you mentioned would, I think, express themselves primarily in rolling resistance.

The sort of feedback of increasing weight requiring more increasing weight for more fuel, larger components, etc. is the bane of the existence of aeronautical engineers.