Pre-Chamber Ignition – Small Spark, Great Effect
Potentials and solutions for using pre-chamber ignition in the passenger car gasoline engine
Pre-chamber ignition is a key technology for successively improving the efficiency of gasoline engines. A proprietary solution developed by IAV shows fuel savings of up to three (passive pre-chamber) and eight percent (active pre-chamber) respectively in the WLTC. At the 39th International Vienna Engine Symposium, IAV presented it to the public and garnered huge interest on the part of clients. Initial projects are already in progress.
What do large, stationary gaseous-fuel engines have in common with engines from Formula- 1 racing cars? Both use pre-chamber ignition! In the case of gas-powered engines, the technology, which has been familiar for decades, provides efficiency levels of around 50 percent. Racing cars benefit from it too, and obtain their enormous power on relatively little fuel. What could be more obvious than to use the process in gasoline engines for regular production cars?
IAV has been working on pre-chamber ignition for many years. It has fitted a single-cylinder engine with the efficiency booster and thoroughly tested it. The idea: in the cylinder head, where the spark plug is located, a small space is separated off which accounts for about two thirds of compression volume and is segregated from the main combustion chamber by a perforated cap. In the case of passive pre-chamber ignition, the mixture enters the pre-chamber through these apertures during the compression stroke and is ignited by the spark plug located therein. With active pre-chamber ignition, the pre-chamber has an additional fuel metering facility (e.g. injection or cycle valve) which produces a stoichiometric mixture at the spark plug. This variant is particularly well suited to engines with high dilution rates.
Stoichiometric operation benefits from low knocking tendency
Following ignition in the pre-chamber, the pressure inside it quickly rises, with spark energy multiplying by several order of magnitudes and several torch jets shooting into the main combustion chamber. “This leads to multiple places of ignition outside the center of the combustion chamber and, in general, brings about rapid combustion and a lower knock tendency in gasoline engines”, explains Marc Sens, Senior Vice President of Advance Gasoline Powertrain Development at IAV. “This makes it possible to increase their geometric compression, significantly improving their efficiency.” Pre-chamber ignition is of particular interest for engines with very high dilution rates which are a key option in connection with powertrain hybridization. “In future, the combustion engine will in most cases be operated at higher loads where avoiding knock as well as real-gas and wall heat losses limit efficiency”, says Dr. Emanuel Binder, development engineer in Marc Sens’ section. “The lean-burn process represents a major development step, making it possible to increase gasoline engine efficiency from today’s volume production maximum of 38 percent at best to 43 percent and more.”
Reliable and rapid combustion as well as low NOx emissions for lean-burn engines
The poor ignition and slow burn-through of the mixture as well as excessively high NOx emissions have so far stood in the way of using leanburn engines. These obstacles are now removed by pre-chamber ignition: Even with highly diluted mixtures – whether with air or with exhaust gas – high ignition energy guarantees that reliable and rapid combustion takes place everywhere. In the case of air dilution, homogeneous lean mixtures with air/fuel ratios well over two can be ignited and efficiently combusted, permitting NOx emissions close to the detection limits. The combination of pre-chamber ignition, lean-burn engines and hybridization could therefore noticeably reduce the fuel consumption of gasoline-powered vehicles.
Engines operating on the Miller or Atkinson principle also benefit from pre-chamber ignition. Their usual lack of turbulence in the combustion chamber is compensated for by the additional turbulence of the jets from the pre-chamber. This permits reliable and rapid combustion here as well, and these combustion processes can demonstrate their benefits (e.g. lower wall heat losses) to the full. “Using pre-chamber ignition, we can boost the efficiency not only of today’s engines but also of new technologies”, Sens says. “Successively, it will allow us to enhance the gasoline engine.”
The potential that pre-chamber ignition harbors is demonstrated by tests on a single-cylinder engine operated with passive or active pre-chamber ignition. With the passive option, fuel consumption in the WLTC fell by up to three percent as a result of the higher compression, and further efficiency improvements could well be possible. Active pre-chamber ignition even produced savings of up to eight percent in the WLTC. “These results are highly promising”, Binder reports. “And we see further potential for optimization by factoring pre-chamber ignition into designing the combustion chamber of new engines.”
Integrated pre-chamber systems are of interest
IAV’s pre-chamber ignition solution is distinguished by a number of unusual design features. Part of the pre-chamber is integrated into the cylinder head and the existing cooling system, and requires no special spark plugs. The cap with the outlet bores, in contrast, is designed as a separate component. The active variant additionally injects a mixture of fuel and air into the pre-chamber at a pressure of five to ten bar whereas other solutions use fuel only or a second fuel under very high pressure. “Prepared outside the pre-chamber, the mixture reduces the chemical losses in the pre-chamber, wall wetting and the emission of hydrocarbons and particulate matter”, Sens reports. The passive option also sets itself apart from other models. It works reliably even at low loads and temperatures, refuting fears that the process will not function at all engine operating points. Consequently, there is no longer anything to stand in the way of using pre-chamber ignition – and not only in gaspowered engines and Formula 1.