Here’s the latest reader question, along with my reply!
Culley asks: If a pick-up is traveling at 60mph and all eight cylinders are running, then the amount of energy required to maintain the 60mph is the amount of energy being used, right? If you cut back to four cylinders running how does that magically make it to were the pick up now requires less energy to maintain 60mph? If the eight cylinders were wasting energy where was that energy going, was the torque converter loosing efficiency? If the eight cylinders were using more energy than necessary, then doesn’t it stand to reason that the truck would not maintain 60mph but would instead be speeding up?
My reply: This requires some parsing. If an engine is equipped with cylinder deactivation (also called variable displacement; e.g., V8/6/4) all of the pistons are still going up and down inside the cylinders because it’s not physically possible to disconnect them from the crankshaft. All that’s happened is fuel delivery to some of the cylinders has been turned off during low-load/coasting and steady-state cruising.
The pistons pump air – and do not fire. Because the injectors aren’t spraying any fuel into those “shut down” cylinders. It doesn’t take much energy to keep a vehicle rolling once it’s rolling.
This is how fuel is saved.
It’s not much – from the perspective of the vehicle’s owner. Typically 1-3 MPG, best case.
It’d never be worth the cost/complexity were it not for CAFE – Corporate Average Fuel Economy – the mandatory fleet averages each automaker must meet else be fined, these fines adding to the cost of the “non-compliant” cars and thereby making them less attractive to buyers.
A 1-3 MPG improvement per vehicle factored over a fleet of vehicles – all the models of that type sold in a year – is a very big deal to the car company, which is why these systems are common. Also automated stop/start – ASS – and direct injection.
Uncle fatwas – we pay.
How much is “saving gas” worth to you?
Got a question about cars – or anything else? Click on the “ask Eric” link and send ’em in!
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