IDLE CONJECTURE

June 20, 2012 § Leave a comment

A recent news story in the News and Observer relates an automobile accident at the RDU airport.  The driver is reported to have left his car idling, and left the vehicle to pick up an incoming passenger, secure in the belief that the car was in a stationary state.  One presumes he thought that it was set to the “park” setting on the gear box.  Well, apparently it was not and it started to move.  Noting this he jumped in and inadvertently hit the accelerator rather than the brake and hit someone, injuring her.

Our point, and I realize you the reader were desperately attempting to discern this, is not what he did, but what he did not do.  He did not switch off the engine.  This is more common than is not.  Who has not seen the line of cars picking up kids at the YMCA or some such establishment, each one of them idling away (of course with a hybrid such as the Prius that is not the case simply because that decision is taken out of their hands; stationary hybrid engines stop by design)?  To study the phenomenon, we shall first normalize for weather and assume that keeping the air conditioning on was not the driver.  Even without actual data I think we can safely assume that this happens in all manner of weather.

Otherwise environmentally conscious people do this, so what is going on?  Getting past sheer laziness or carelessness, most are likely still steeped in an old dogma: starting an engine uses an inordinate amount of energy and it is better to keep it running.  There used to be an old rule of thumb of thirty seconds as a breakeven time.  If you took a poll of otherwise highly educated friends regarding the breakeven time you are likely to encounter a range of numbers ranging up to a minute.  Even with young people who became drivers well after the era of carburetors, the dogma is entrenched, or they have simply not thought about it.

In the 70’s and 80’s cars were switched from carburetors to fuel injection.  The former bled in the air for combustion by suction, while the latter pressurized the mixture precisely and in fine droplets that were injected.  This allowed for a leaner fuel mix and the cooling effect in the cylinder helped improve the effective octane rating slightly.  Electronically timed injection happened in the early 90’s.  So if your car is younger than twenty years, it has this feature. 

The significance is that there is virtually no penalty for stop and start.  To test for the breakeven time experiments were conducted by engineers of the Florida chapter of the American Society of Mechanical Engineers on V6 powered vehicles.  They concluded that 5 seconds of idling equated to the fuel required to start the engine (6 if the air conditioner was on).  Does this mean we should stop the engine at red lights?  Probably not.  The time to take an automatic out of park and start the engine is several seconds.  Also, even if one did do this flawlessly, the car was likely not designed for such frequent restarts and something would wear out.  The fuel economy part is correct but something else may give.  In hybrids the start is with an electric motor, which is designed for the purpose and highly reliable.

But there is no debate when it comes to idling while waiting to pick someone up.  When picking up kids it could also be a teaching moment.  As a bonus, the young staff herding the kids will also lead better respiratory lives as a result.

Vikram Rao

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