On the Way to 45 Percent Efficiency

The bar is set high: in future, passenger car engines will have to achieve up to 45 percent efficiency to meet the globally planned CO₂and emission limits. It has already been decided that the limit value valid in Europe from 2020 will be decreased by 15 percent for 2025 and even by 30 percent for 2030. These ambitious targets can only be met by combining several complementary technologies, including the Miller cycle, higher compression, cooled EGR and homogeneous lean combustion. The specific technology components to be selected essentially depend on the drive concept respectively the level of electrification in each case. There is therefore a need for solutions that can be put to beneficial use in as many of these drive concepts as possible.

Pre-chamber ignition creates the prerequisites for gradually integrating efficiency technologies in the combustion engine, making an evolutionary contribution in various topologies. Although the Miller cycle already brings about a clear increase in efficiency, a highly specific design of the combustion process is needed to counteract the associated drawbacks. But this does not always work completely, with some of the introduced measures bringing other drawbacks. This is exactly where a combination with pre-chamber ignition offers a major advantage, with the possibility of compensating for the typical Miller cycle drawbacks. Compared to combustion methods with conventional spark plug and Miller cycle, this brings a further two to three percent in fuel savings. Increasing geometric compression with the associated fuel saving of two to three percent can only be achieved by introducing pre-chamber ignition due to the major knock advantage this provides. If a combustion engine is operated with externally cooled EGR, pre-chamber ignition once again compensates for process-related drawbacks and achieves a further reduction of one to two percent in fuel consumption compared to combustion methods with conventional spark plugs. It is active pre-chamber ignition that makes homogeneous lean combustion methods possible in the first place, providing far more energy for ignition of the mixture than conventional spark plugs. In Aachen, IAV will be presenting a near-production exhibit, as well as using the lecture on “Pre-Chamber Ignition and Promising Complementary Technologies” to take a detailed look at the potential of pre-chamber ignition in combination with the other efficiency technologies.

“Pre-chamber ignition is a key technology for the economical engines of the future”, says Marc Sens, Senior Vice President of Powertrain Gasoline Engine Advance Engineering at IAV. The really interesting thing about pre-chamber ignition is that it permits evolutionary enhancement of the combustion engine. For example, it helps to improve knock-behavior at the start; fuel consumption can then be further reduced at a later stage with the Miller cycle or homogeneous lean mixtures. “We still have to deal with some technical challenges, such as ensuring the engine will start reliably even at temperatures below freezing point, or designing a pre-chamber that works reliably under the most diverse operating conditions”, explains Sens. “But IAV has already presented promising solutions in this respect that are currently going through the development process.”

Besides pre-chamber ignition, in Aachen IAV will also be presenting a base engine with three cylinders and 1.1 liters displacement available in several power output ratings, making it ideal for a CO₂-optimized fleet. The engine is available as a 72-kW naturally aspirated engine for smaller models, and as supercharged versions with 90 or 120 kW for larger vehicles. It has been designed with a particularly long stroke to support modern combustion processes. The base engine is optimized in terms of fuel consumption and costs and comes with small package demands and a modular structure. For example, it can be supplemented with valve lift changeover, balancer shafts or electric accessory units. Efficiency can be further enhanced with technologies such as 3D-printed pistons and advanced thermal management. Another option comprises a new model for all secondary air systems. It combines all individual systems used today in the engine and in the vehicle, thus managing with far fewer components.

In Aachen, IAV will also be showing a DHT (Dedicated Hybrid Transmission) planetary transmission that is ideal for combination with the base engine. It has just two planetary gear sets and four shift elements. The electric motor consists of a permanent magnet synchronous motor. Depending on the specific design, it supplies outputs between 25 and 190 kilowatts and torques from 150 to 500 Newton meters. It is so well integrated in the transmission that it needs little package, making it possible for the hybrid powertrain to fit in small vehicles. The transmission is rated for a combined drive torque of maximum 750 Newton meters from the combustion engine and electric motor. Thanks to its maximum input speed of 7,000 rpm, the integrated electric motor can be used in a broad operating range.

“We at IAV always think in terms of complete powertrains”, says Marc Sens. “With the combination of base engine and DHT transmission together with pre-chamber ignition as key technology for higher efficiency, we can offer our customers viable solutions for their specific fleets.”

At the site of the spark plug in the cylinder head, a small chamber with a perforated cap is separated from the main combustion chamber. With passive pre-chamber ignition, the compression stroke pushes the mixture through these openings into the pre-chamber where it is ignited by the spark plug. In active pre-chamber ignition, a separate fuel metering device is added that ensures a constant supply of stoichiometric mixture in the pre-chamber, even during lean operation in the main combustion chamber.

More information about pre-chamber ignition can also be found in this video