Many car enthusiasts don’t even think about how exactly they shut down the power unit after a trip. The habitual turning of the key in the ignition or pressing the “Stop” button seems like actions that do not require any technical training or deep knowledge. However, modern internal combustion engines, especially those equipped with turbocharging systems, require more careful attention when stopping. Incorrect actions can lead to accelerated wear of turbine bearings, coke formation in lubrication channels and other hidden defects.
In this article, we will analyze in detail the physics of the processes occurring in the engine when it stops, and answer the question of whether it is necessary to allow the engine to “cool down” before turning off the ignition. The features of operating vehicles with various types of power plants, including hybrid systems, where the stopping process may differ from the classical scheme, will also be considered. Understanding these nuances will help extend the life of expensive components of your vehicle.
Physics of the process: what happens in the engine when it stops
The moment you turn off the ignition, the supply of spark and fuel stops, but the inertial forces continue to act. The crankshaft, flywheel and related components do not stop instantly. Inertial move may last for several seconds, at which time the oil pressure in the system drops sharply as the oil pump stops working. This is a critical moment for all rubbing pairs, especially for the turbocharger, which can continue to rotate at enormous speed due to inertia.
If the engine was running under load, the temperature of the components reaches maximum values. In turbocharged engines, the turbine shaft can heat up to 800-900 degrees Celsius and higher. Cooling occurs due to the circulation of oil and antifreeze. An abrupt cessation of fluid circulation leads to the fact that the oil remaining in the bearings begins to coke. The resulting solid deposits clog the channels, which eventually causes the turbine to fail.
⚠️ Attention: An abrupt stop of a hot turbocharged engine after intense driving on the highway can reduce the life of the turbocharger by 30-40% due to coking of the oil in the bearing unit.
In naturally aspirated engines, the processes are less aggressive, but the principle remains the same. When metal parts heat up, they expand, and when they cool quickly, microscopic deformations can occur. This is especially true for cylinder heads made of aluminum alloys, which have a different coefficient of thermal expansion compared to steel liners or valves.
Should the engine be allowed to idle?
There is a widespread belief that any car must be warmed up before a trip and “blowed through” after it. In the case of stopping the engine, the “warm-up” rule is transformed into the need to idle to stabilize temperature conditions. This is especially true for diesel units and gasoline engines with a turbine. The time of such operation usually ranges from 30 seconds to 2 minutes, depending on the degree of load before stopping.
For modern naturally aspirated gasoline engines used in the urban cycle, prolonged idling after a trip is often not required. The cooling and lubrication systems of modern cars are designed to cope with thermal loads using standard methods. Moreover, prolonged idling can lead to hypothermia engine in winter or, conversely, to local overheating due to insufficient pump performance at low speeds.
It is important to consider the type of fuel and engine design. Diesel engines running at low speeds do not produce enough heat to warm up the diesel particulate filter (DPF) unless the system has initiated regeneration. However, after active driving, it is useful to let a diesel engine, like gasoline, run for a minute to equalize the temperatures of the pistons and liners.
Features of stopping turbocharged engines
Turbocharged engines require the most delicate handling. A turbocharger is a device that operates under extreme conditions. The rotation speed of the turbine shaft can reach 200,000 revolutions per minute or more. Lubrication is carried out by oil under pressure, and it is this that removes the bulk of the heat from the bearing assembly. If you turn off the engine immediately after overtaking on the highway, the oil in the turbine assembly will boil, forming carbon deposits.
Many modern cars are equipped with forced cooling systems for the turbine after the ignition is turned off. Electric pumps continue to circulate antifreeze, removing heat from the hot turbocharger housing. The presence of such a system can be checked in the technical documentation for the model Volvo, BMW or VAG. If your car has such a system, then there is no need for manual “blowing”.
However, if you are not sure about the availability of additional cooling systems or your car does not belong to the premium segment of recent years of production, it is better to play it safe. Let the engine idle for about 60-90 seconds after vigorous driving. This time is enough to reduce the temperature of the exhaust gases and stabilize the thermal state.
The influence of the driving mode on the stopping procedure
Driving style directly dictates the algorithm of actions when parking. If you were moving in dense city traffic at a speed of 40-60 km/h, constantly stuck in traffic jams and traffic lights, then the engine temperature is stable and moderate. In that case instant stop will not harm either naturally aspirated or turbocharged engines. Thermal loads are distributed evenly, and there is no sharp temperature gradient.
A completely different situation arises after driving on the highway, overtaking or towing a trailer. In these modes, the engine and transmission operate at the limit of their thermal capabilities. Stopping immediately after leaving the highway is a thermal shock for the units. The metal quickly cools down and contracts, which can lead to a leak in the gaskets, especially the cylinder head gaskets.
It is also worth considering the ambient temperature. In winter, at subzero temperatures, cooling processes occur faster, and the risk of moisture condensation in the exhaust system is higher. In the summer, when it’s hot, the risk of overheating and boiling of technical fluids in the “dead zones” of the engine increases many times over. Therefore, in the summer, after active driving, the pause before turning off the ignition should be a little longer.
| Engine type | Riding mode | Recommended idle time | Risk when stopping suddenly |
|---|---|---|---|
| Atmospheric gasoline | City/Highway | 0 - 30 sec | Minimum |
| Turbo petrol | City | 30 - 60 sec | Medium (coking) |
| Turbo petrol | Track/Sport | 2 - 3 min | High (turbine output) |
| Diesel (with DPF) | Any | 1 - 2 min | Medium (particulate filter) |
Technical nuances: Start/Stop button and ignition key
In cars with a classic ignition key, the stopping process is completely controlled by the driver. You decide when to turn the key to the “0” position. In cars with the system Start/Stop Engine (button) logic may be different. Some manufacturers program the electronic engine control system (ECM) to ignore the first button press if the speed is too high, or, conversely, to turn off the engine instantly for the sake of the environment and fuel economy.
Owners of cars with a button should be careful. Often the system does not warn you about the need to let the engine run. Here the responsibility lies entirely with the driver. If you have just pulled into the garage after a dynamic drive, do not immediately reach for the button. Wait until the tachometer needle drops to a stable minimum value, usually this 600-800 rpm.
There is also a nuance with automatic engine shutdown systems in parking lots (Start-Stop function in traffic jams). If the car stalled on its own at a traffic light, and then you drove on, this is normal mode. But if you stopped at a gas station or at a barrier, and the system automatically turned off the engine, but it was hot, this is acceptable, since modern algorithms take into account the temperature factor. However, you shouldn’t rely on electronics 100% of the time, especially on older cars.
⚠️ Attention: On cars with diesel engines and a particulate filter, do not turn off the engine if the DPF regeneration indicator is on on the dashboard. The filter cleaning process must be completed completely, otherwise the system may become clogged and enter emergency mode.
Common driver mistakes when turning off a car
One of the most common mistakes is turning off the engine with power consumers turned on. Headlights, climate control, heated seats and powerful audio systems put a strain on the on-board network. When the ignition is turned off, voltage surges may occur that negatively affect the electronics and the battery. The correct algorithm would be to turn off all consumers before the final shutdown of the engine.
The second mistake is ignoring extraneous sounds. If, after turning off the ignition, you hear loud knocks of metal on metal or a strong hissing sound, this may indicate a malfunction of the cooling system (antifreeze boiling) or detonation (dieseling). In such cases, the engine should not be stopped abruptly if it is still running, or immediate diagnosis is required if it is already stopped.
The third mistake is typical for the winter period. Drivers turn off the engine without allowing it to run for a couple of minutes after warming up, but forget that the condensate in the exhaust system must evaporate. If you turn off a cold engine immediately after a short trip, water will remain inside the muffler, which will freeze in winter, blocking the exhaust gas outlet, or cause metal corrosion.
It is also worth mentioning manual transmissions. Before turning off the engine, you must ensure that the vehicle is securely secured. On a slope, always use the parking brake and engage a gear (first or reverse) to be on the safe side. An attempt to turn off the engine in neutral without using the handbrake on a slope can lead to uncontrolled movement of the vehicle.
Frequently asked questions (FAQ)
Should I wait until the cooling fan starts running before turning off the engine?
In most modern cars this is not required or even desirable. The electronics itself controls the fan, which can continue to operate even after the ignition is turned off (if the battery charge and design allow). Waiting for the fan to turn on can waste fuel and time. It is enough to let the engine run for 1-2 minutes at idle.
Is it harmful to turn off the engine in high gear?
You can turn off the engine only after the car has come to a complete stop and the gearbox is in neutral (N) or pressing the clutch. An attempt to turn off the ignition while driving or in gear without pressing the clutch will result in a jerk and possible damage to the transmission or starter components the next time you try to start.
Can frequent abrupt engine shutdowns lead to water hammer?
No, water hammer is associated with water entering the cylinders through the intake manifold, and not with the procedure of turning off the ignition. However, an abrupt stop of a hot engine can lead to deformation of the cylinder head, which in the future may cause antifreeze to enter the cylinders, but this is a consequence of overheating, and not water hammer in the classical sense.
How to properly turn off a car with gas equipment (gas equipment)?
Cars with gas equipment of the 4th generation and higher usually switch to gasoline themselves before stopping if they run out of gas. There are no special requirements for the shutdown procedure, but it is recommended that there is gasoline left in the injector rail for the next start, so let the system switch itself or force it before parking.