How does the engine speed affect the exhaust emissions?


Lexicon> letter L> idle

Definition: the operation of an engine with no load

English: engine idling

Categories: Vehicles, Basic Terms, Engines and Power Plants

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 01.06.2013; last change: 03/14/2020


If an internal combustion engine is not needed for a short time, it is often left in the Neutral keep walking. Otherwise it would have to be restarted for another use, which puts a strain on the starter and the battery and can lead to increased levels of pollutants in the exhaust gas (see below).

In the vehicle, the engine drives the alternator (a generator for generating electricity) even when the vehicle is idling. Modern alternators can still provide a certain amount of power even when idling, despite the low speed; so can z. B. the battery can be charged. The generated electrical power is, however, very low compared to the fuel consumption, i. H. the efficiency is very low (at most a few percent). Another useful effect can be the generation of heat for heating the vehicle, which also results in very high energy losses. The compressor of an air conditioning system can also be driven, although the primary energy consumed is far more than the amount of heat withdrawn from the vehicle.

Idle speed

A typical car engine needs an idle speed of the order of 800 revolutions per minute, i.e. approx. 13 revolutions per second. (In normal operation, the speed is significantly higher - usually at least 1500 revolutions per minute, with high power under certain circumstances even more than 5000 rpm.) The idling speed is usually kept constant by a special controller. In the warm-up phase (after a cold start) it can be set a little higher under certain circumstances.

Consumption and pollutant emissions when idling

Did you know that a car engine consumes so much energy when it is idling that you could easily heat an entire house with it?

The fuel consumption of an engine when idling is quite considerable: with a typical car engine in the order of 10 kW (based on the primary energy), e.g. B. Much more than the heating system in a well-insulated detached house has to do on the coldest days, and much more than the consumption of a gas stove. This burns around a liter of fuel per hour - often completely useless. If the idle speed is set unnecessarily high, consumption can also be higher. The high amount of waste heat to be dissipated can mean that the fan for engine cooling occasionally has to go into operation, especially when the engine is idling.

Frictional losses are not the main problem.

The surprisingly high idle consumption results only to a small extent from friction losses in the engine, and much more from the inefficiency of the drive principle when idling. This is especially true for gasoline engines (Otto engines); here the throttle valve must remain almost closed when idling (in order to achieve the necessary combustion air ratio), so that high throttle losses occur. In addition, incomplete combustion of the fuel can occur, as can the relatively high loss of heat through the walls of the combustion chamber.

The pollutant emissions of a gasoline engine (Otto engine) when idling are relatively high: there are hardly any nitrogen oxides, but v. a. significant amounts of toxic carbon monoxide and unburned hydrocarbons in gasoline engines. When the catalytic converter has reached operating temperature, it can break down a large part of these pollutants into carbon dioxide and water vapor. Shortly after a cold start, when the engine's raw emissions are even higher, the catalytic converter is ineffective, so that very toxic (including carcinogenic) exhaust gases leave the exhaust. Interestingly, the pollutant emissions can be reduced with a light load on the engine - e.g. B. by the alternator - decrease despite increased fuel consumption; the combustion is worst when the engine is completely unloaded.

Diesel engines can also produce considerable emissions when idling - largely depending on the engine technology in question, and again increasingly during the warm-up phase.

As far as climate-damaging carbon dioxide emissions are concerned, these are only dependent on fuel consumption. In other words, they are already reduced in the event of very brief shutdowns, while the quantities of the above-mentioned toxic pollutants are less reduced or may even increase.

From what length of a break is it worth switching off the engine?

The question arises as to the length of an operating break from which a car engine should be switched off. As far as fuel consumption is concerned, there is a saving of just a few seconds when the vehicle is switched off, as restarting the vehicle hardly consumes any additional fuel. The picture is somewhat different for pollutant emissions, as the starting process is often unfavorable in this regard (at least when using simple technology); in the case of a longer break, the catalytic converter can also cool down to such an extent that it is briefly less effective afterwards. Nevertheless, it is generally recommended to switch off the petrol engine if the machine is not used for more than 15 seconds. In the case of diesel engines, a slightly longer interval of e.g. B. 20 to 30 seconds make more sense. In large diesel engines such. B. buses, this minimum time can even be significantly higher, z. B. at a minute.

Automatic start-stop

In most situations, vehicles with an automatic start-stop system are switched off automatically as soon as the engine is not needed - for example when the engine is idling and the clutch pedal is released. The technology is then usually optimized in such a way that the disadvantages of switching off are minimized, and starting is again carried out automatically. In city traffic, this technology can achieve considerable fuel savings.

Fuel cut-off

Most car engines today have an overrun cut-off, i.e. H. the fuel supply is interrupted in overrun mode (overrun mode). This avoids idling consumption and increases the effect of the engine brake.

Hybrid drives

For vehicles with hybrid drive (at least for Full hybrids) idling is avoided as much as possible - even while driving, even without overrun, when little or no drive power is required. During the warm-up phase or in very cold weather (with a high demand for heating), idle operation can remain necessary, but then some power is often generated in order to charge the hybrid battery. This strategy leads to the operating temperature being reached more quickly and, above all, to reduced pollutant emissions.

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See also: combustion engine, overrun fuel cut-off, engine brake, automatic start-stop, energy consumption, fuel, hybrid drive, alternator
as well as other articles in the categories vehicles, basic concepts, engines and power plants