CNG Motor Design Achieves High Efficiency

July 1, 2019 | 05:43
A 4-cylinder 2 Liter CNG motor on the test stand at the Empa. Image: Empa.
A 4-cylinder 2 Liter CNG motor on the test stand at the Empa. Image: Empa.
In the search for a cleaner vehicle fuel for combustion engines, compressed natural gas (CNG) looks to be a promising alternative to oil-based fuels. Together with electric and hydrogen-powered propulsion systems, they play an important role in the SCCER Mobility (Swiss Competence Center for Energy research), led by the ETH in Zurich. Reprocessed biogas or synthetic methane ("e-gas") vehicles produce very low CO2 emissions. Synthetic methane can be generated using temporary excess renewable electricity and CO2. Treated biogas and synthetic methane gas can be combined as necessary to produce a fuel with an octane rating up to 130 which is a significantly higher anti-knock rating than gasoline and is why CNG is particularly suitable to power internal combustion engines. Under sustained high load conditions, such as highway driving, gas-powered vehicles can achieve higher efficiencies than petrol engines.

There is also further potential to exploit the high knocking resistance of methane by specifically engineering the engine design. CNG engines used in today's passenger vehicles are typically only lightly-modified gasoline engines and therefore not optimized for operation with methane. The EU GasOn project started work in 2015 to exploit the potential of methane, researchers from ETH Zurich and Empa also participated in a work package led by Volkswagen Group Research.

 
The pre-chamber layout. Image: Empa

An engine optimized to run on CNG

A highly efficient combustion process was developed and used in the design of a 2 liter CNG engine: A lean gas mixture is first ignited in a thimble-sized, flow-controlled prechamber. In the ETH-Laboratory for Aerothermochemistry and Combustion Systems, fundamental experiments on the ignition behavior in the pre-chamber and the resulting hot gas expanding into the main combustion chamber were investigated. With the help of this data, mathmatical models were further developed in order to be able to characterize the processes in detail by means of computer simulations.

The results allowed Volkswagen Group Research to optimize the design of the prechamber and main combustion chamber. At Empa, an engine has been designed to allow further investigation of the combustion characteristics. An engine management system developed by the Institute for Dynamic Systems and Control Technology at the ETH Zurich was used. This system provides overall engine control and allows adaptation of operating conditions according to research findings.
 
Cross-section through the cylinder head: The pre-chamber is in the centre showing the spark plug and gas injector. Image: Empa.

Efficiency record for passenger car engines

Compared with previous designs the new CNG engine with pre-chamber combustion leads to the fuel consumption saving of about 20% (based on the WLTP standard consumption of a mid-range car). The peak efficiency of the new engine design was over 45%, with efficiencies of over 40% recorded over wider operating conditions. Such values ​​are currently only achieved by significantly larger engines, such as those found in commercial vehicles, in fixed installations or in marine applications. This is a new record for car engine efficiency. By comparison, gasoline engines have maximum efficiencies of 35 to 40% across their optimal working range. In the GasOn project, the engine exhaust post-treatment has not yet been processed; there is still a need for research here due to the lean burning process.

Overall, it has been shown that a relatively small CNG engines can achieve similar efficiencies to much larger diesel engines. They also run easily on any fuel ratio made up of renewable biogenic and synthetic methane, achieving very low CO2 emissions. The participating vehicle manufacturers are now clarifying how the results of the GasOn project can be transferred to production vehicles.
 
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