Most Efficient Engine Warm-up Occurs on the Go

Should engines be warmed before heading out of the driveway? It all depends on a number of factors, but in most cases, the simplest and best answer is no. A warm engine certainly provides more heat in the passenger compartment, but there are better ways to warm an engine than to sit idle.

Internal combustion engines turn only a fraction of the energy in a gallon of fuel into useful work. In a well-tuned engine, most of that lost energy is referred to as waste heat. Waste heat must be carried away by coolant to keep engine systems at proper operating temperatures. At startup, that waste heat brings oil and engine temperatures up to optimum levels. However, it can take a long time to warm an engine that is doing nothing but turning its own crankshaft.

Bring engines up to operating temperature quickly and efficiently by loading them gently and at low engine speeds as soon as the engine starts and runs smoothly. In most cases this means starting the engine and heading out the driveway immediately, but don’t be in a hurry when starting a cold engine. Accelerate gently for the first couple of minutes to allow engine oil to warm before placing a significant load on the engine. The warm-up period will be shorter and the engine will reach its most fuel efficient operating conditions more quickly.

Immediately at startup and especially on very cold days, the vaporization of fuel is an important limiting factor in the operation of spark ignition engines. Gasoline must be vaporized in order to burn. When the engine is cold, quite a bit of the fuel either condenses on cold engine parts or otherwise just fails to vaporize. In order to get enough vaporized fuel into the combustion chamber, more fuel must be metered. The unavoidable consequence is that for the first few minutes of operation, the engine wastes more fuel than it will after it is warmed. But this effect is small compared to the amount of fuel wasted by idling. All of the fuel used during idling is wasted.

Don’t let a quest for fuel efficiency cloud good judgment or cloud your view of the road. Sometimes it is not practical to get on the road until the engine is producing enough heat to keep the windshield free of ice. Safety should always be a top priority, but comfort can be sacrificed.

Idling during frequent stops is also not necessary even on cold days. An engine retains quite a bit of heat and will stay plenty warm without leaving the engine running. If in doubt, always follow manufacturer’s recommendations for cold weather operation and put safety first.

Bill Casady
Agricultural Engineering
573.882.4370
CasadyW@missouri.edu