Why aren't gasoline engines equipped with a turbocharger?


Lexicon> Letter T> Turbocharging

Definition: a method of increasing the performance or efficiency of an internal combustion engine

English: turbocharging, supercharging

Category: Vehicles

Author: Dr. Rüdiger Paschotta

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Original creation: 09/05/2014; last change: 03/14/2020

URL: https://www.energie-lexikon.info/turboaufladen.html

The Turbocharging is first of all a method to increase the performance of an internal combustion engine. In this case, combustion air is fed to the engine during the intake phase (first cycle) with the aid of a compressor under increased pressure, so that the amount of fuel can also be increased accordingly. The compressor is driven by a turbine that uses the energy of the exhaust gas flow. (The term Exhaust gas turbocharger underlines this additionally.) The compressor and the exhaust gas turbine sit on a common shaft, which can reach a very high speed during operation. This assembly is called turbocharger designated. Turbocharged engines are called Turbo engines or as supercharged engines designated. In contrast, there are naturally aspirated engines that are not supercharged.

The so-called degree of delivery of the engine can be increased by turbocharging, if the theoretically possible amount of air is related to the conditions outside the entire engine assembly (i.e. with normal pressure). If, on the other hand, it is related to the conditions directly in front of the inlet valve, the turbocharging essentially does not increase the degree of delivery.

Today it is common to replace a turbocharger with a so-called Intercooler to complete. Because of the compression of the supplied air, its temperature rises considerably, and this is disadvantageous for various reasons. A charge air cooler can significantly reduce this increase in temperature and thus contribute to both an increase in performance and an improvement in efficiency.

The drive of the turbocharger does not directly take some of the drive power from the engine, as would be the case with a compressor. (In the case of a supercharged engine, the compressor is driven directly by the combustion engine.) However, the back pressure in the exhaust system tends to increase, so that the losses increase in the fourth stroke. This can be compensated to some extent by increasing the pipe cross-sections in the exhaust system and minimizing the pressure loss in the muffler.

The primary purpose of turbocharging is to increase engine performance for a given cubic capacity. Compared to increasing the displacement, which in principle would be the simpler method of increasing performance, turbocharging requires a smaller increase in the weight of the drive.

Turbocharging can also be used to achieve a given engine output with a smaller and therefore lighter engine. This is of particular interest in the case of vehicles - especially in the case of aircraft.

Does turbocharging increase or decrease fuel consumption?

As a rule, the energy efficiency will decrease, i. H. fuel consumption will increase when the engine of a vehicle of a given cubic capacity is turbocharged. However, if downsizing is used at the same time so as not to ultimately increase the performance, increased efficiency or reduced fuel consumption is often possible. This results on the one hand from the fact that the efficiency of the engine (especially in partial load operation) can increase, and on the other hand from the reduction in vehicle weight.

Since the increase in performance naturally goes hand in hand with an increase in the mechanical and thermal stress on various parts, turbocharging requires various additional measures in order to achieve a long service life.

A good mastery of strongly changing load conditions is not easy with turbocharging.

Especially when using vehicles, the engines have to cope with heavily and quickly changing load conditions. This is technically not easy in connection with turbocharging. A particularly well-known problem is the so-called Turbo lag, d. H. a delayed onset of power when accelerating; The turbocharger can only develop its full effect when the exhaust gas flow has risen accordingly as a result of the increased output. On the other hand, the effect of the turbocharger should not be too strong at high speeds; often it then has to be with a bypass (a Wastegate), which must be appropriately controlled. Such problems can be greatly reduced with various measures, which, however, require considerable technical effort (also in connection with the control of various components). For example, idlers are often used in the turbocharger, the position of which is optimized via a servo drive depending on the current conditions. In some cases, an additional compressor driven directly by the engine (e.g. a Roots blower) is also used to increase the torque at low speeds. Other options are the support of the turbocharger with a powerful electric motor and the use of an additional electrically driven compressor (“eBooster”) in front of the turbocharger. The significant increase in the complexity of the drive system in connection with the use of components that are sometimes highly stressed also increases the risk of defects.

A special case is its use in aircraft engines that are supposed to provide high performance even at great altitudes. At high altitudes, the density of the air is quite low, so that it becomes more difficult to fill the cylinders well without turbocharging. In some aircraft, turbocharging was essentially only used to compensate for the pressure drop at higher altitudes. However, it is also possible to achieve generally higher boost pressures for the benefit of increased engine output.

Turbochargers can be used in both gasoline and diesel engines, although in some cases different technical aspects are relevant, which are discussed below.

Turbo charging of gasoline engines

The fact that turbocharging increases the tendency to knock limits the effect of their use in gasoline engines.

A fundamental problem with turbocharging in gasoline engines (gasoline engines) is that the temperature rise increases during the compression phase, which increases the tendency for the fuel-air mixture to spontaneously ignite. This spontaneous ignition leads to what is known as knocking, which must be avoided because of the heavy mechanical load it causes on the engine.

A common method of preventing knocking despite turbocharging is to reduce the compression ratio. Unfortunately, this is disadvantageous for the efficiency, since it also reduces the expansion ratio in the working phase of the engine. In principle, a fuel with increased knock resistance could also be used, but a further increase in the compression ratio would actually be desirable with this too. An efficient intercooler reduces the problem a little. Other methods are the additional injection of water or a water-methanol mixture in order to cool the air supplied. You can also optimize the timing, but here too there are conflicting goals. Incidentally, gasoline direct injection is beneficial, as the cooling effect due to the evaporation heat extracted from the fuel is optimal.

Turbo gasoline engines are often more dependent on full-load enrichment, which unfortunately has a very negative effect on fuel consumption and exhaust quality.

A simple and frequently used method is the so-called Full load enrichment, d. H. the use of a certain excess of fuel, which lowers the combustion temperature somewhat, when operating at high loads. However, because of the incomplete combustion, this results in increased fuel consumption and sharply rising pollutant emissions. Therefore this method should be used as little as possible. The article on full load enrichment has more details.

A particularly effective, but also complex, method for suppressing the tendency to knock in a turbo engine is water injection. It effectively increases the charge air cooling and enables a significant increase in engine performance on the one hand and efficiency on the other. The further development of this method has been started in recent years and its introduction in series vehicles is to be expected soon.

Despite the problem mentioned that turbocharging generally requires a somewhat reduced compression ratio and tends to require more frequent use of full-load enrichment, energy efficiency can often be significantly improved by turbocharging in conjunction with downsizing. The smaller motor suffers less from throttling losses than a larger motor in partial load operation. In addition, there is the effect of the reduced vehicle weight. However, this advantage can be turned into the opposite if you frequently drive at full throttle, so that the full load enrichment has a harmful effect.

Due to the relatively high exhaust gas temperature of gasoline engines, the turbocharger is exposed to high thermal loads. This makes the use of high temperature resistant materials in connection with an optimized construction necessary. Such problems can be mastered with today's technology, but the effort is considerable.

Reliable ignition of the combustion is more difficult with high boost pressure and especially in combination with lean operation. In this case it may be necessary to use a reinforced ignition system with a higher ignition voltage and higher ignition energy.

In gasoline engines, turbocharging is particularly effective in the medium to high speed range, but hardly at low speeds. The elasticity of the supercharged engine is therefore less than that of a non-supercharged engine with the same maximum output (i.e. higher displacement).

Turbo charging of diesel engines

Compared to gasoline engines, there is more to be said for the use of turbocharging in diesel engines:

In diesel engines, turbocharging is particularly effective and, at the same time, technically easier to control.
  • The problem of knocking does not exist in diesel engines. (In spite of this, the compression ratio is usually reduced somewhat even in diesel engines when turbocharging is used.) Therefore, the possible increase in output is usually more pronounced than with gasoline engines.
  • The higher power-to-weight ratio of diesel engines, which is particularly relevant for vehicles, can be reduced quite effectively by turbocharging.
  • Since the exhaust gas temperatures in diesel engines are significantly lower than in gasoline engines for various reasons (higher compression ratio, higher combustion air ratio), the thermal load on the turbocharger is significantly lower.
  • In diesel engines, turbocharging improves the so-called elasticity of the engine, i. H. it also improves the torque at low to medium speeds.
Turbocharging can also be very useful in stationary engines - but not for exactly the same reasons as in vehicles.

For these reasons, turbocharging is much more common in diesel engines than in gasoline engines. This applies to the field of vehicles, but also to stationary engines. In the latter case, the reduction of the drive weight is of less importance, but mostly they are large engines, where the efficiency and specific costs of turbochargers are lower than those of small engines. An essential aspect of stationary engines is that they do not have to go through constantly changing operating conditions, which greatly simplifies the use of a turbocharger.

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See also: combustion engine, gasoline engine, diesel engine, naturally aspirated engine, knocking in gasoline engines, displacement, degree of delivery, fuel injection, water injection, full-load enrichment, downsizing of combustion engines
as well as other articles in the category vehicles