Warming up a parked engine stopped being useful in 1993

It is seven in the morning and the thermometer reads two below zero. Out in the street there is a car running with nobody in it. The vapour grows behind the bumper while its owner is still indoors; a few metres away, a neighbour has repeated the ritual. Ten minutes a day like this add up to around 61 hours a year with the engine burning fuel and the car in the same spot.

The habit had a mechanical explanation. Carburettor engines needed the choke during a cold start. That device enriched the mixture of air and petrol to prevent unstable running or the car stalling. Electronic fuel injection handed that work to the engine control unit, which takes readings from the sensors and adjusts the fuel automatically. Its spread coincided with the Euro 1 standard, compulsory for new cars from 1993, and with the widespread adoption of the lambda sensor and the three-way catalytic converter.

It is worth saying out loud the belief this overturns, because hardly anyone has revisited it since they learned it: that a cold engine suffers if you start it and drive off, and that minutes spent parked are a way of looking after it. With a carburettor they served that purpose. Today those minutes coincide precisely with the stretch in which the engine works at its worst: enriched mixture, oil still spreading and a cold converter, all three at once and with the car going nowhere.

In a modern petrol car you can start up and spend 20 to 30 seconds fastening your seat belt and adjusting the seat. The oil starts circulating the moment the engine turns over; during those seconds it finishes reaching the areas it has to protect. Then it is best to set off and drive gently for the first two or three kilometres. About 2,000 revs works as a practical reference, though load matters as much as engine speed: crawling along at low revs with the accelerator buried, especially uphill, also asks too much of a cold engine. The right gear and a moderate pedal matter more than obeying a figure as if it were a universal limit.

Revving while stationary makes the delicate moment worse. The control unit uses an enriched mixture during a cold start and some of the petrol can go unburnt. That fuel washes away the film of oil on the cylinder walls and ends up diluting the lubricant. A calm departure lets the whole thing gain temperature under a moderate load. Idling, it may need fifteen or twenty minutes to reach working temperature; driving gently, about five.

The exhaust system warms up too. The catalytic converter starts treating gases effectively at around 250-300 degrees. While it stays cold, a larger share of the emissions leaves with little treatment. A long wait at idle stretches that phase out. The effect weighs most on two-kilometre trips, where the engine barely has time to settle and the water produced by combustion can condense in the exhaust and mix with the oil. Three short trips in a day repeat three enriched starts. Combining errands gives the oil, the exhaust and the converter more time inside their normal range.

The exceptions fit into a single paragraph. An old carburettor car may need the choke and a few moments to settle, depending on its particular system. In an old diesel, the glow plug warning light asks you to wait until it goes out before starting; that is seconds, with the engine still off. At twenty below zero, a block heater plugged into the mains can warm the engine without burning fuel, a solution that belongs to extreme climates.

What those two cars in the street are spending can be worked out. An idling engine uses between 0.6 and 1.2 litres an hour. Ten minutes a day over a year come to some 3,650 minutes, around 61 hours. Taking 0.8 litres an hour, a mid-range value within that interval, that gives about 49 litres used without moving. At 1.55 euros a litre, the bill comes to 75.95 euros: around 76 euros a year. Put it in fuel-pump terms and it is almost the whole tank of a small car, burnt without leaving the parking space. The exact result changes with the engine, the temperature and the accessories, but the petrol used during that wait produces heat and gases without providing any travel.

How long a stop lasts raises another calculation. After about ten seconds, idling has already used more fuel than restarting requires. That figure on its own is not advice to turn the key by hand at every traffic light. Efficiency guidance uses a practical threshold of more than a minute for long waits, such as a level crossing, a pick-up or a motionless queue. Below that time, the saving is small against the added wear on the starter motor and the battery.

Cars with automatic stop-start handle traffic lights on their own. They carry batteries and starter motors built to withstand many more cycles than conventional components. In a vehicle without that system, switching off by hand at every urban halt multiplies work those parts may not be sized for. The one-minute reference stays reserved for long pauses.

The ritual survives for a reason that has nothing to do with mechanics: it is cold inside the car. Someone leaving it running outside the house is almost never protecting the engine, they are waiting for warm air to come out. The two things are separate, and the second is also solved faster by driving: the engine produces heat far more quickly under load than parked, so the cabin warms up sooner on the way to work than waiting outside the front door.

That leaves the most visible reason to wait: the windscreen. If it is misted up on the inside, the air conditioning removes moisture and the heater raises the temperature of the air directed at the glass. They can run together while you get ready to leave. If the ice is on the outside, hot water can cause a sudden change of temperature and crack the glass. A scraper recovers the whole surface without depending on ten minutes of running engine. Driving while looking through a small defrosted patch is still dangerous even if the cabin is already warm.

The heater takes a moment because it uses the engine's heat, and turning it up hard from the first second can slightly delay warm-up. That does not mean you have to sit there cold: set off gently, aim the air at the windscreen and adjust the temperature as the heat arrives. The air conditioning can stay on to dry the air. When you next look over at your neighbour, his car may still be giving off vapour in the same parking space while yours is already moving and reaching its working temperature sooner.

Put a scraper in the boot tonight. That one gesture prepares your next departure better than leaving the engine running for ten minutes outside the house.