Most people think that aggressive driving gives you bad gas mileage. But those who obsess about miles per gallon ("hypermilers") have some counterintuitive techniques. One of them is "pulse and glide", which calls for almost flooring the accelerator. Why does it work?
Gas engines have a throttle that prevents air from freely flowing into the engine (unless you're at maximum acceleration, when the throttle is wide open). The engine needs the correct fuel-to-air ratio, and this is how it's achieved. But when the throttle is holding back the air, the engine is forced to work to pull in the air it needs. The engine is acting like a pump, consuming energy. Holding the throttle wide open minimizes those pumping losses.
This is why big engines get lousy gas mileage: most of the time their throttles are almost completely closed and they are literally sucking air.
The bad news is, you can't really take advantage of this technique with an automatic transmission. In order to make it work, you have to be able to choose what gear you're in, and an automatic takes that choice away from you. Besides, let's face it, you can only floor it when there's nobody in front of you. Hell is other people, isn't it?
Another reason larger engines have poorer fuel economy: frictional losses. A higher displacement engine has larger surfaces moving against each other and consuming power. It's a small but real effect.
ReplyDeleteAs a general rule, the less vacuum an engine is pulling the lower its pumping losses. This is one of the reasons for higher efficiency of Diesels, though the higher energy content of diesel fuel is a major contributor to their miles per gallon advantage over throttled gas engines.
Every few years some PR genius 'invents' a 100 MPG car and takes it on a nationwide tour of credulous local news programs. This usually turns out to be an industrial diesel tuned and geared to run wide open at highway speeds. It will have pathetic acceleration, laughable driving dynamics, filthy exhaust and true 100 MPG straight line efficiency. This is invariably held up as evidence that the world's dozens of furiously competing automakers have paradoxically conspired to suppress some high mileage technology that would have given an unbeatable market advantage to any of them that had brought it to production.
What the auto industry really needs is a light Diesel of about 15 - 20HP output. Install that in a plug-in hybrid for battery charging only and optimize it to run the generator at highest possible efficiency. Freed from the need to have low emissions at a variety of speeds, and with a high and narrow power peak, an engine like that could approach 100 MPG efficiency. I hope the first gen of plug in hybrids will be configured that way. It could make our petroleum resources go a long way.
The Prius and other strong hybrids are approaching your 15-20 hp engine idea. The challenge is that the electric parts become larger, heavier, and more expensive. The current Prius is roughly 50% electric, and it does run its gas engine in a more efficient mode than other cars do.
ReplyDeleteAnother challenge is that using a gas engine to drive the wheels directly is more efficient than putting the power through a generator, then a battery, then an electric motor. So there is a tradeoff.
I picked the 15 - 20 HP figure based on a recollection of the horsepower needs of large cars at steady state highway speed. Road and Track used to publish the horsepower consumed at 65 MPH for all vehicles tested. I recall a Rolls Royce was highest and a Civic was lowest.
ReplyDeleteAny idea what the power loss percentage is in the cycle of generating, storing and using electricity in a hybrid?
According to this wikipedia article, battery charging and discharging efficiencies are 70-85%. That link actually presents an interesting efficiency comparison, starting from the point of combustion. The high figure (85%) is probably for letting the vehicle choose its own charging cycle. The low end is probably when the user chooses the discharge rate. So for a serial hybrid, you'd have to take each of those hits once.
ReplyDeleteSupercapacitors, interestingly, have charging and discharging efficiencies of around 98%, as you'd expect. Unfortunately they can currently only store about 1% of the energy per unit mass of lithium-ion batteries.
The self-discharge rate of lithium ion batteries in good condition is 5-10% per month, which for a car is probably irrelevant.