[教學] Idling time

carwing

By Jim Kerr


How long should you idle your vehicle before you drive away? Thereseem to be many opinions out there and there is even legislation insome parts of the country that limit the amount of time you are allowedto idle your vehicle. This usually becomes an issue in the winter, asmany Canadians want to climb into a warm vehicle; there are alsodrivers who idle their vehicles in the summer to cool the interior downwith the air conditioning. So how long do you really need to idle avehicle so it will operate properly?
Tires, wheel bearings, seals, differential and transmission oils andthe engine all need to be warmed up before the vehicle operates itsbest. Tires and transmissions can take several kilometres of driving towarm them up, depending on outside temperatures, so idling the enginedoesn’t help. You need to drive the vehicle, but before that, youprobably want to warm the engine up a little.
Dramatic changes occur when starting a cold engine: combustionchamber temperatures go from below zero to over 800 C in an eye-blink.Parts expand rapidly with the temperature changes, but not all at thesame rate. The engine block stays cold for several minutes, while thecylinder head and pistons warm up much faster. Modern gasket andfastener technology allows parts to expand at different rates, whilepiston designs help control their expansion. One advantage of warmingan engine up is to allow the parts to expand uniformly before placingheavy driving loads on them. This reduces wear and stresses.
Oil also needs to be warmed up. At temperatures near zero, the oilis thick but still pours easily. When temperatures drop to –20 C, theoil pours more like molasses. Winter grade oils such as 5W-20 and 5W-30viscosity will flow much easier, so they lubricate better at coldtemperatures. When outside temperatures drop to –40, as they do inparts of the country, you can leave fingerprints in summer grade oils.Winter grade oils help but still flow very slowly until warmed up.During the warm-up period, parts may not receive the best lubricationso you want to keep engine speeds and loads low.
Okay, so how long does it take to warm up the engine? On a zerodegree day, oil will be flowing and you can start driving slowly in 30seconds to a minute. If you have to get out on the highway or freewayimmediately, let it warm up a couple more minutes. At temperatures of–20 or below, you are best to let the engine warm up two to threeminutes before driving, but then drive slowly for a few kilometres tolet the transmission and other rotating parts warm up too. Usually whentemperatures drop below zero, the ability of the heater to clear thewindshield of frost determines the warm up time. Once it is clear, youare set to go.
There are some exceptions where you should warm up the vehiclelonger. Turbocharged engines require additional lubrication for thehigh speed bushings inside the turbocharger. The engine temperatureshould be starting to rise before you start driving so the oil willflow freely to these bushings. Diesel engines also take longer to warmup. Idle speeds on diesel engines are controlled by the amount of fuelinjected. At idle, there is very little fuel burned and not much heatproduced, so it takes longer to warm up. You may even have to drive fora little while to warm up a diesel.
Block heaters are a good way to keep your engine warm whentemperatures drop, but they are not really needed above about –18 C.Even at colder temperatures, plugging in a block heater for more thanfour hours is a waste of electricity, as the engine won’t getsignificantly warmer if plugged in longer.
Thermostats are important to getting the engine up to operatingtemperature quickly. The thermostat stops coolant from circulatingthrough the radiator while the coolant is cold. If even a littlecoolant bypasses the thermostat because it is sticking or opening toosoon, the engine takes much longer to warm up. This causes the computerto inject more fuel in the cold engine, so fuel economy drops andengine wear increases. Most engine wear occurs during cold starts.
Enough of winter: think of summer. It is nice to let the engine idleand the air conditioning cool the interior, but it isn’t necessary. Ifthe engine has been started for 30 seconds, you are ready to go driving.

whats_up

very good actricle