A single-stroke engine is possible

Two-stroke engine

Lexicon> Letter Z> Two-stroke engine

Definition: an internal combustion engine (usually in the form of a reciprocating piston engine) in which work is done with every downward movement of the piston

More general terms: internal combustion engine

English: two-stroke engine

Category: Engines and Power Plants

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: May 3rd, 2015; last change: 10/22/2020

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

A Two-stroke engine is an internal combustion engine in the form of a reciprocating piston engine, which completes only two strokes per working cycle, namely within only one upward and downward movement of the piston, instead of the four-stroke engine with four strokes within two upward and downward movements. The main advantages of this approach are the following:

The two-stroke principle has several basic advantages - but also significant disadvantages.
  • In relation to the cubic capacity, a higher output can be achieved, since a working cycle driving the crankshaft is possible with every revolution of the crankshaft instead of just every second revolution as in the four-stroke engine. This (together with the mixture lubrication and generally often simpler design) enables more compact designs and a reduced weight for a given engine power. However, the liter capacity is less than doubled, as various disadvantages (see below) also occur.
  • A more compact design also leads to the operating temperature being reached more quickly after a cold start.
  • In engines with few cylinders, smoother running (with less strong vibrations) is possible, and the mechanical load on the crankshaft drive for a given output is lower.
  • The energy losses due to friction are lower than with the four-stroke engine. However, a higher degree of efficiency is only achieved with large diesel engines, while two-stroke gasoline engines are usually less efficient than four-stroke engines due to various imperfections.
  • Depending on the design, the manufacturing costs for such engines can be considerably lower than for four-stroke engines.

However, these advantages are also offset by a large number of disadvantages, which are discussed further below. The extent of the advantages and disadvantages depends heavily on the respective design.

There are extremely different types of two-stroke engines with correspondingly different properties, for example with regard to energy efficiency and exhaust quality.

There are a variety of designs for two-stroke engines. Many of them work on the basic principle of the gasoline engine, while others are diesel engines. Small engines powered by gasoline (with a few kilowatts of power), for example for small motorized two-wheelers, outboard engines and work equipment such as lawnmowers, leaf blowers, power saws and snow blowers, and very large two-stroke diesel engines, for example for propelling ships, are particularly widespread. Less common are gasoline-powered two-stroke engines with tens of kilowatts for larger boats and snowmobiles. Today, two-stroke engines are practically of no importance for driving cars and trucks, except in a few countries with extremely lax emissions legislation. In the 1990s, efforts were made to achieve advantages with newly developed two-stroke engines, particularly with regard to fuel consumption, but these seem to have failed due to various difficulties (primarily in connection with exhaust gas quality and possibly with complexity and service life) be. In addition, particularly in the case of diesel engines, the difference in performance between two-stroke and four-stroke engines has decreased due to the widespread use of turbocharging.

Functional principle of the two-stroke engine

The basic principle of a simply built (slot-controlled) small two-stroke engine for operation with gasoline is shown in Figure 1 and explained below:

The gas exchange is accomplished in a relatively tricky way in the two-stroke engine.
  • When the working piston moves upwards, the fuel-air mixture is compressed in the combustion chamber above the piston. At the same time, the negative pressure below the piston causes a new fuel-air mixture, which is generated, for example, with a carburetor, to be sucked into the crankcase.
  • A little before the top dead center is reached, the combustion is started with the aid of the spark plug. The pressure in the combustion chamber increases sharply and the piston is driven by it during the subsequent downward movement.
  • At a certain position, the piston releases the outlet slot so that some of the burned mixture can be expelled as exhaust gas into the exhaust.
  • Below the piston, the previously sucked in fuel-air mixture is compressed and passed into the combustion chamber via an overflow channel as soon as the corresponding inlet slot is cleared by the piston. The inflow of the mixture leads to a further displacement of the exhaust gas. A backflow to the carburetor is prevented, for example, by a diaphragm valve. Modern diaphragm valves still work reliably even at very high engine speeds.

The gas exchange is often supported by vibrations in the gases in the exhaust tract and in the intake tract. With a suitable design of the exhaust tract, for example, a resonant oscillation can occur there, which prevents an excessive loss of fresh gas. (The successful use of such gas dynamics, however, is usually limited to a certain speed range, except if the geometry of the exhaust tract is adjusted depending on the speed, e.g. via controllable valves.) The usual one is also helpful Reverse flush with tangential alignment of the inlet ducts, which acts against a direct overflow to the exhaust.

There are quite different ways of controlling the gas exchange in two-stroke engines.

The above explanations apply to a simple design with piston edge control: the inlet as well as the outlet is controlled by the piston opening or closing corresponding slots. (In addition, a diaphragm valve serves as a non-return valve.) With other types, there is a rotary slide control instead or control by another, phase-shifted, oscillating piston. In large two-stroke diesel engines in particular, control by means of poppet valves is used, similar to a four-stroke gasoline engine, at least for the outlet or for the inlet and outlet (with Reverse rinse).

The usual methods of gas exchange in small engines (especially with piston edge control) result on the one hand in a simple and inexpensive construction (e.g. without valves and their complex drive and control mechanisms), but on the other hand have considerable disadvantages:

A complete removal of the exhaust gas and a complete filling of the combustion chamber with mixture is hardly possible with a simple two-stroke engine.
  • In order to be able to remove the exhaust gas completely enough, the outlet slot must be opened long before the bottom dead center is reached. A significant part of the expansion work is lost due to the low expansion ratio.
  • In terms of manufacturing costs, it is advantageous to use the space below the piston as a pump for the fresh gas (principle of Crank chamber wash pump). However, this leads to high losses of lubricating oil - especially in the case of conventional mixed lubrication (see below). In addition, after a cold start, a lot of fuel can condense on cold engine parts in the crankcase. To compensate for this, the mixture can be enriched even more during the warm-up phase, but such measures are often difficult to dose correctly.
Flushing losses are a major reason for poor exhaust gas values ​​and high fuel consumption in two-stroke engines.
  • The fresh gas flows into the combustion chamber while the outlet slot is still open. This means that there is a risk that a considerable part of the fresh gas will go straight into the exhaust, which on the one hand means wasting fuel (Flushing losses) and, on the other hand, makes a major contribution to the emission of unburned hydrocarbons, i.e. to a deterioration in the quality of the exhaust gas. Especially for engines in vehicles that have to work with a wide variety of loads and speeds, it is difficult to design the gas exchange well with such means.
Unintentionally strong exhaust gas recirculation during operation with a low load can be the main cause of uneven engine operation and incomplete combustion.
  • If the supply of the mixture is throttled in partial load operation, the proportion of exhaust gas in the mixture increases accordingly. Such exhaust gas recirculation (to a very high degree) has certain advantages (for example a reduction in throttle losses), but also disadvantages, above all incomplete combustion with correspondingly higher pollutant emissions and poorer fuel economy. The typical out-of-round (“uncultivated”) running behavior in the lower partial load range and especially when idling is a consequence of this: Particularly incomplete combustion often occurs, but in the next work cycle the proportion of gasoline vapor and oxygen is higher than normal due to the remaining residual gas so that the following combustion delivers significantly more heat and thus drive energy.
  • Because the cylinder wall contains slots, there is an additional mechanical load on the piston rings and thus an increased risk of piston seizure (i.e. catastrophic destruction of the engine), especially in operating conditions with insufficient lubrication.
  • An asymmetrical load on the piston as a result of the uneven cylinder filling is also often unfavorable. This can negate the benefit of the lower mean effective pressure for a given engine output and increase wear and friction losses.

Various methods can be used to improve the gas exchange, which, however, usually also lead to an increase in the technical effort:

A number of disadvantages of simple two-stroke engines can be mitigated or avoided with additional measures - which of course increases the technical effort accordingly.
  • An improved control of the gas exchange enables control with rotary valves, which must be driven accordingly. (For example, some two-stroke engines have improved exhaust control through the use of a rotary roller valve that reduces the opening of the exhaust duct in some operating states.) Ideally, the function should be optimized depending on the respective operating conditions (especially load and speed). The same applies to the use of valves as in the four-stroke engine.
  • A more targeted fuel supply is possible with the help of direct fuel injection. (In this case, the engine draws in pure air and the fuel is only supplied shortly before ignition.) This can reduce scavenging losses and improve cylinder filling. This technology is used, for example, on some motorcycles, primarily to achieve a higher liter output.
  • Improved cylinder filling can be achieved with the help of turbocharging, for example in the form of turbocharging or with a Roots blower, often combined with direct current purging.

Simpler measures, for example the optimization of piston and combustion chamber shapes, offer less potential for improvement.


Large valve-controlled engines usually have a pressure-circulating lubrication, as is common with four-stroke engines. However, this method excludes the use of the crankcase as a pump chamber for the mixture, as this would quickly lead to a dilution of the lubricating oil. A separate blower is therefore required for the gas exchange.

Mixture lubrication is a technically simple and low-maintenance solution, which, however, has considerable disadvantages and limitations.

In the case of small gasoline-powered two-stroke engines, however, there is one Mixture lubrication (or Mixture lubrication) the most common variant. Here a certain amount of lubricating oil is added to the fuel, for example in a ratio of 1:50, i. H. 1/50 = 2% of the two-stroke mixture consists of lubricating oil. (This admixture is usually made before refueling, i.e. you have to fill up with a two-stroke mixture with a suitable oil content and not pure gasoline.) No oil change is then necessary, which reduces maintenance costs. This type of lubrication also works with different orientations of the engine, which is particularly important with hand-held tools. The lubricating oil is burned after its use (Consumption lubrication) - but quite incomplete, which leads to high emissions of unburned hydrocarbons and often also to disruptive deposits in the engine and in the exhaust system. Of course, this type of lubrication cannot be combined with direct injection.

Longer overrun operation can lead to total loss in two-stroke engines with mixed lubrication!

Another problem with mixture lubrication is the dependency of the lubrication on the fuel supply. In principle, it is true to use less lubricating oil with a reduced load. However, during prolonged pushing operation (for example when driving downhill) the lubrication may be inadequate, which can lead to engine damage. (An overrun fuel cut-off would not be applicable in this situation anyway.) This is one of the reasons why there are systems with Separate lubricationin which the lubricating oil can be carried in a separate tank and can then be better dosed. The lubricating oil can be brought to the cylinder liners with the aid of one or more oil pumps, for example. In this way, sufficient lubrication can be achieved in all situations, while the average oil consumption is reduced. Even with this, however, the oil consumption is far higher than with a four-stroke engine.

The requirements for lubricating oils for two-stroke engines are significantly different from those for four-stroke engines. For example, it is important that the oil produces as little smoke and malodorous substances as possible when it is burned and that it causes as few deposits as possible in the engine and the exhaust system. Other aspects are good solubility in fuel and increased corrosion protection for the engine, since oxygen is constantly supplied to the crankcase. On the other hand, the temperature dependency of the viscosity and the dispersibility are of little importance.

Exhaust quality and fuel consumption

Petrol engines

The widespread gasoline-powered two-stroke small engines with a simple design (piston edge control and mixture lubrication) usually have a poor exhaust quality, especially since they usually do not have any exhaust gas cleaning system. Although they do not emit too much nitrogen oxides, they do emit large amounts of carbon monoxide and unburned hydrocarbons, the latter being caused partly by the flushing losses and partly by the mixture lubrication, and in summer they contribute significantly to ozone smog. Frequent operation with a very rich mixture (→Full load enrichment) and very imprecise control of the combustion air ratio contribute to this problem. Unburned oil components also contribute to particulate emissions (fine dust), which are known as Blue smoke are often clearly visible. Various hydrocarbons also cause the distinctive odor of such exhaust gases.

The exhaust gases from many two-stroke engines are very toxic and therefore a health hazard for people who have to inhale such exhaust gases. Fuel consumption is also often disproportionately high

Small two-wheelers powered by such engines emit far more toxic pollutants per kilometer than a car. (The current emission limits are much less stringent than for cars; the article on carbon monoxide gives a numerical example.) In cities where mopeds are frequently used, they create a large proportion of the particulate matter in the air, even if they represent a fraction of the total fuel consumption cause. Because of their low efficiency, they also cause a disproportionately high fuel consumption - even with mopeds with 40 km / h often even more than a small car, although the required engine power is much lower.

The mentioned disadvantages of small two-stroke engines are of course negligible if such engines only achieve a few operating hours per year. For example, a snow blower is often only used for a few hours within a winter, so that the effort for a better engine (and for increased maintenance, e.g. for the oil change required in a four-stroke engine) would be disproportionately high. This applies to many hand-held implements. One problem, however, is that people working with such devices are often exposed to very high levels of local air pollution. The emissions problem is therefore less of an ecological than a health problem.

Exhaust gas catalytic converters can only be used to a limited extent in two-stroke engines, since a well-regulated combustion air ratio would be required for good effectiveness.The conventional control with a lambda probe does not work well if the combustion is not stable, but keeps showing failures, as often occurs with two-stroke engines, especially in the lower partial load range. A high proportion of unburned hydrocarbons in the raw exhaust gas can also lead to a high thermal load on the catalytic converter, in which the hydrocarbons are then oxidized. In addition, the loading of the catalytic converter with residues of lubricating oil can impair its effectiveness.

Where can you easily replace two-stroke engines with four-stroke engines?

Wherever possible, two-stroke engines should be replaced by four-stroke engines, above all in order to achieve a massive reduction in air pollution. In some areas, such as motorized two-wheelers, lawnmowers and boat engines, there is no compelling reason to continue using two-stroke engines. (There is even an increasing possibility of using an electric drive.) It is more difficult with hand-held tools, which with a four-stroke engine would be considerably heavier and more unwieldy and where portable batteries would not have enough capacity. The use of an electrical device with a cable can of course also be a solution - with certain practical disadvantages due to the cable, but also considerable advantages: low weight, hardly any maintenance, no fuel supply, much less noise, no pollution from toxic exhaust gases, etc.

Diesel engines

Completely different conditions exist with two-stroke diesel engines, which have a significantly different design and operating conditions. The efficiency of such engines can be well above 50% at full load, so that such two-stroke engines are among the most energy-efficient heat engines. The exhaust gas problem is also completely different here:

The main problems with marine diesel engines are operation with low-quality heavy fuel oil and the lack of particle filters and denitrification systems.
  • A main problem is that ship engines in particular are often operated with cheap, but very inferior heavy fuel oil for reasons of cost. Above all, it has a high sulfur content (several percent), which leads to correspondingly high emissions of toxic sulfur dioxide. This problem can of course be solved by using higher quality diesel fuel.
  • In addition, there are particle emissions (fine dust) as a result of incomplete combustion, against which a soot particle filter can be used effectively. In the absence of appropriate regulations z. B. for marine engines, however, this has seldom happened so far.
  • In addition, nitrogen oxides are produced, which could (but mostly not) be eliminated by a denitrification system on a ship, for example.

Noise emissions

Devices and vehicles with two-stroke engines are often noticeable due to their particularly unpleasant noise emissions.

Many vehicles and devices (e.g. chainsaws and leaf blowers) that are equipped with a two-stroke engine are noticeable because of their very annoying noise emissions - especially because of a sharp, sawing noise under heavy loads. Such noise emissions are mainly caused by sharp pressure surges that are caused when the exhaust gas escapes through the outlet slot and partly also by the air being sucked in at an inlet slot. In principle, such noise emissions could be dampened with optimized exhaust systems, for example, but this is difficult to achieve in practice, especially with small devices that are supposed to be very compact. This problem does not exist in large two-stroke diesel engines, especially since mostly valves are used here that do not generate such strong pressure surges from the outset.

Two-stroke engines are relatively quiet when idling, especially since they vibrate less than four-stroke engines, and are acoustically very easily distinguishable from four-stroke engines due to the often very restless (“nervous”) running of the engine.

In general, most four-stroke engines have a rather “beefy” sound with strong emissions, especially at low frequencies, while two-stroke engines are dominated by high-frequency emissions (at full load).

Switching between two-stroke and four-stroke operation

In principle, it is possible to build an engine that works either as a two-stroke engine or as a four-stroke engine, depending on the required power and speed. This could be advantageous, for example, in order to achieve a greatly increased torque in the lower speed range without requiring a large displacement (→Downsizing). Of course, such an engine would not be simpler than a four-stroke engine, on the contrary, it would be significantly more complex; in particular, a fully variable valve control is required. It is therefore unclear whether this idea will prevail. But there are already a number of patents on this.

Questions and comments from readers


A rarely good and understandable report about 2T engines. Only your sentence, “Where possible, two-stroke engines should be replaced by four-stroke engines” is, in my opinion, out of date.

The Euro 2 level has been in force for the type approval of motorcycles since April 2003. The limit values ​​are specified in EU Directive 2002/51 / EC. While with Euro 1 there was still a differentiation of the exhaust gas limit values ​​for motorcycles according to engine type in two-stroke and four-stroke models, this separation is removed from Euro 2 in favor of a differentiation according to engine size.

The new EU regulation 168/2013 for motorcycles and mopeds has been in force since January 2016. Now mopeds must also comply with the stricter emission values ​​that apply to motorcycles. The currently valid EURO standard is only achieved by 2T and 4T with electronically controlled carburetors or injection.

If I have not misunderstood anything due to the current EU regulations, from 2003 motorcycles with 2T engines (later in the case of mopeds) do not blow more harmful exhaust gases into the air than the corresponding 4T engines.

Even with snowmobiles and outboards, there is no longer any difference in emission limit values ​​between 2 and 4-stroke engines.

Answer from the author:

First of all, my comment did not only refer to two-wheelers, but e.g. B. also on outboard motors for boats and various tools.

As for two-wheelers: If an emissions standard such as B. Euro 2 is designed in such a way that it can still be fulfilled with cheaply built two-stroke engines, it is more than slack. Even Euro 4 according to EU regulation 168/2013 allows z. B. still 630 mg / km hydrocarbons (HC), to be compared with 100 mg / km for cars with petrol engines according to Euro 4. Why is it allowed that a micro vehicle for the transport of one person is allowed to emit many times more HC than a car? Just so that cheap two-stroke engines are allowed. If the two-stroke engine had not been invented at all, these limit values ​​would certainly be stricter.

Probably the result will be that cheap devices with two-stroke engines just meet this terrible norm, while four-stroke engines can easily go below the limit values.

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See also: internal combustion engine, reciprocating piston engine, four-stroke engine, gasoline engine, diesel engine, exhaust gas quality, exhaust gas recirculation, exhaust gas catalytic converter
as well as other items in the engine and power plant category