Starting over
Dr. Edward P. Becker | TLT Automotive Tribology June 2017
You’re stuck in traffic. Should you shut off your engine to save fuel?
The fuel required to restart a car engine is approximately equal to the fuel required to idle for 10 seconds.
© Can Stock Photo / pazham
Idle: adjective 1. (of a person) avoiding work; lazy. 2. without purpose or effect; pointless.
THE MAJOR DISADVANTAGE OF THE INTERNAL COMBUSTION ENGINE compared to other prime movers (steam and electric) is that the internal combustion engine is not self-starting. Early gasoline and diesel vehicles were equipped with a rather large hand crank, and the energy to start the engine was provided by the motorist. Hand cranks gradually disappeared from vehicles with the invention of the electric starter. It is a rather amusing anachronism that, to this day, when we turn the key to the Start position (or press the Start button) we still say we are “cranking the engine.”
Another problem, however, is that the engine must be disconnected from the wheels during starting and whenever the vehicle is stopped. Modern vehicles can only be started when the gear shift lever is in the park or neutral position and while the engine is running the clutch (on a manual transmission) or torque converter (on an automatic) to allow the engine to run without turning the wheels. The situation of the engine running while the vehicle is stopped is most appropriately called idling. Consuming fuel without moving is antithetical to the very concept of the automobile.
The fuel lost to idling is quite significant. The U.S. Department of Energy (DOE) estimates that six billion gallons of fuel are wasted idling in the U.S. annually (DOE/CHO-AC-06CH11357-1502, May 2015). So how long do you need to be stopped for the fuel to restart the engine to be less than would be used to idle? Should you do it?
The first question is the easier one to answer. The same U.S. DOE report states that, for a fully warmed-up engine, the fuel to restart is approximately equal to the fuel required to idle for 10 seconds. If the vehicle is stationary for longer, turning off the engine saves fuel.
As for the second question, we need to consider such things as traffic flow and safety. In stop-and-go traffic, the time spent not moving is variable and difficult to predict, so shutting off the engine is probably not a good idea, as restarting time will further delay traffic. Red lights in urban areas typically last fewer than 60 seconds, and your vehicle probably won’t remain stationary for that entire period, so shutting down may be impractical in this situation also.
More favorable scenarios are when you are parked waiting for a passenger or at railroad crossing, where the time is likely to greatly exceed the 10 seconds requirement, and you can safely restart once you need to enter the traffic flow. Of course, this will fall far short of the maximum possible fuel savings.
A better solution is for the vehicle to shut down and restart automatically, and that is how the so-called mild hybrid vehicles work. The first such system in mass production was General Motor’s Belted Alternator Starter (BAS) system in 2007, later refined into their current e-Assist package. The electronic control system restarts the engine much more quickly and reliably than the driver could, so traffic is not impeded. Estimates of fuel savings vary, but maximum savings occur in city stop-and-go driving, and this strategy is a feature of all parallel hybrid vehicles.
If you do decide to shut your engine down at long stops, don’t worry about wearing out your starter or its components. Modern designs and materials are sufficiently robust to handle the extra duty!
Ed Becker is an STLE Fellow and past president. He is president of Friction & Wear Solutions, LLC, in Brighton, Mich., and can be reached through his website at www.frictionandwearsolutions.com.