Bosch is leading a consortium in a German government-funded project to develop a direct injection system for monovalent natural gas engines—i.e., engines that run exclusively on CNG. Compared with present systems that use manifold gas injection, a direct injection system for natural gas could deliver as much as 60% more torque at low rpm, and offer the prospect of an even more dynamic driving experience in the CNG cars of the future.
Today’s CNG vehicles are generally bivalent, running on gasoline and CNG with engines designed for gasoline direct injection. For CNG operation, they are fitted with an additional manifold injection system for methane. However, because methane behaves differently from gasoline when injected directly, it is important to optimize the combustion process for methane.
The problem with this configuration is that neither the combustion process nor the values for efficiency and emissions can be optimized. For this to happen, the CNG—like the gasoline—needs to be injected directly into the combustion chamber.—Dr. Andreas Birkefeld, the project leader from Robert Bosch GmbH
Researchers in an unrelated study of direct injection natural gasoline engines (dual-fuel combustion in CI-engines using RCCI as well as conventional duel-fuel combustion) at Chalmers University (Sweden) noted that methane’s very high octane number allows the use of high compression ratios in spark ignition engines.
The very high octane rating and resulting high resistance to knock is promising for small, supercharged, downsized SI engines. Additional advantages include lower heat losses due to lower combustion temperature, no need for enrichment, no problems with soot, high EGR tolerance and good prospects for stratified combustion. A direct injection stratified natural gas engine could reduce CO2 emissions by 35% compared to a contemporary port injection gasoline engine, the team said.
The Direct4Gas researchers and engineers will design samples of a direct injector that satisfies much higher standards than the manifold injection valves used up to now. It will have to be especially robust, gas-tight, and reliable, and meter the CNG very precisely.
Modifications to the engine itself are to be kept to a minimum, so that the industry can continue using the same components as for gasoline engines.
The project team will equip experimental gas engines with the newly developed injector, and test it in the laboratory and in vehicles. Researchers will also examine mixture formation, ignition, and exhaust-gas treatment and develop specific solutions.
The long-term objective of the consortium of automotive suppliers and automakers is to create the conditions needed for making the technology ready for production, and the project is an important step toward this goal.
The consortium is led by Robert Bosch GmbH. Other partners include Daimler AG and the Research Institute of Automotive Engineering and Vehicle Engines Stuttgart (FKFS). Umicore AG & Co. KG is an associated partner.
Following a resolution of the German Bundestag, Direct4Gas is supported with €3.8 million (US$4.3 million) from the Federal Ministry for Economic Affairs and Energy as part of the “Increasing vehicle powertrain efficiency” initiative. The project started in January 2015 and will run until the end of 2017.
By 2020, newly registered vehicles in the EU will not be permitted to emit more than 95 grams of CO2 per kilometer on average. By 2025, this limit could be even lower. Efficient CNG vehicles can help meet exacting emissions standards, and this not only because they emit less CO2. Emissions of particulate matter are also significantly lower than from gasoline or diesel engines.