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Compact Engine Architecture for Best Fuel Efficiency and High Performance - Challenge or Contradiction

Journal Article
ISSN: 1946-3936, e-ISSN: 1946-3944
Published November 08, 2011 by Society of Automotive Engineers of Japan in Japan
Compact Engine Architecture for Best Fuel Efficiency and High Performance - Challenge or Contradiction
Citation: Hubmann, C., Schoeffmann, W., Friedl, H., and Howlett, M., "Compact Engine Architecture for Best Fuel Efficiency and High Performance - Challenge or Contradiction," SAE Int. J. Engines 5(3):825-837, 2012,
Language: English


The world of automotive engineering shows a clear direction for upcoming development trends. Stringent fleet average fuel consumption targets and CO2 penalties as well as rising fuel prices and the consumer demand to lower operating costs increases the engineering efforts to optimize fuel economy. Passenger car engines have the benefit of higher degree of technology which can be utilized to reach the challenging targets. Variable valve timing, downsizing and turbo charging, direct gasoline injection, highly sophisticated operating strategies and even more electrification are already common technologies in the automotive industry but can not be directly carried over into a motorcycle application. The major differences like very small packaging space, higher rated speeds, higher power density in combination with lower production numbers and product costs do not allow implementation such high of degree of advanced technology into small-engine applications.
With these boundary conditions and the fact that the demand of lowering CO2 emissions is also evident in the small engine market, solutions need to be created which are suitable for compact engine (CE) applications without major impact on the overall engine architecture. Therefore the development trend is focused on measures which can be easily implemented into existing engine design and powertrain packages optimizing:
  • Mechanical efficiency
  • Indicated efficiency
The mechanical efficiency improvement by reduction of friction is dependent on all interfaces with relative movement velocity. Type of bearings (roller bearings vs. plain bearings), surface conditions (type of coatings) and component specifications (piston ring pre-tension) are a few examples of deciding factors for friction reduction.
The major contributor in the indicated efficiency is the combustion. Especially high speed / high power motorcycle engines suffer very often from poor combustion stability and quality in part / low load operating condition which results in rich mixture (Lambda < 1) and therefore bad fuel economy. To increase the combustion quality residual gas content needs to be reduced and charge motion in the combustion chamber increased.
All these above described measures for reducing the CO2 emission of a CE should be introduced into existing models without major changes in the design to have a quick and cost effective implementation into series production.