STORY

How do gas engines differ from diesel engines?

Posted on February 27, 2019 by Lucie Maluck, Images by mtu

What are the differences between diesel and gas engines? This overview provides some answers.
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Gas engines are increasingly taking the place of diesels in a range of areas including off-highway applications. Both types of engine deliver similar performance but gas engines emit less CO2 during the combustion process. So, what precisely are the differences between the two? This overview provides some answers.


Turbochargers

The small differences
The turbochargers used on diesel engines and mobile gas engines are virtually identical. They both feed the engine with the air (and, therefore, oxygen) it needs for combustion. On stationary gas units, the turbocharger has to process both the air and the entire volume of the gas/air mixture and in stationary genset applications, turbochargers are optimized to match full-load conditions because these engines generally operate at full-load.

Mixture cooling

The small differences
Diesel engines have charge-air coolers that cool the air heated in the compressor before it enters the combustion chamber. On gas engines, this function is performed by the mixture cooler. Depending on the application in question, the gas/air mixture is cooled in two stages to around 50° C to 60° C before passing on to the combustion chamber. The thermal energy extracted during the cooling process can be decoupled from the system and fed into a heating system, for example.

Fuel mixture

The small differences
In diesel engines, air is sucked into the combustion chamber where it is compressed to levels that raise its temperature as high as 700C. Injectors then introduce diesel fuel that ignites in the hot air. In stationary gas engines, the air is mixed with fuel gas before it passes through the turbocharger and mixture cooler to the combustion chamber. Mobile gas engines utilize multi-point injection systems. Here, air is routed to the cylinder and gas is introduced just before it enters the combustion chamber. This means that the volume of gas can be flexibly regulated depending on the power required.

Ignition

The small differences
The most obvious difference between diesel and gas engines can be found in the ignition systems. Diesels are self-igniting engines in which high levels of compression cause the diesel/air mixture to ignite spontaneously. Like gasoline-fueled engines, gas engines use a spark generated by a spark plug to ignite the gas/air mixture (the illustration shows the spark plug connectors and cables leading to the cylinder head cover). Diesel injection systems and extraneous ignition systems on gas engines both need suitable control concepts that determine factors such as, for example, injection timing and duration (diesels) or ignition point and energy (gas).

Knock control

The small differences
Diesel fuels adhere to precise specifications and deliver highly consistent levels of quality for efficient engine set-up. However, the constituents in gaseous fuels vary and this affects combustion. For example, different gases have different methane numbers (similar to octane ratings for gasoline) that indicate the proportional mixture of an equivalent fuel consisting of methane and hydrogen. If the methane number is too low, inefficient spark ignition and other uncontrolled combustion processes can occur in the combustion chamber. These generate ‘engine knock’ and gas engines need to be controlled to deal with the phenomenon. Stationary gas engines use vibration sensors to identify knock whilst pressure sensors do the same on mobile gas engines. Consequently, ignition timing is adjusted as an initial reaction and engine power can be reduced as a second step. In extreme cases, the engine can be shut down to prevent damage.

Throttle flaps

The small differences
On both diesels and mobile gas units the formation of the fuel mixture is controlled flexibly for each ignition sequence in order to influence engine power. Stationary gas engines use a premixed gas/air mixture that remains constant and engine power is influenced by using throttle flaps to regulate the flow of the mixture. Mobile gas engines also have throttle flaps but these regulate the pressure of the mixture entering the cylinder.

Point of contact

Dr. Christoph Heinz
Phone:
+49 7541 90 4661
E-mail:

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