Clean Air Two-Stroke for Utility Engine Applications Demonstration

This page updated October 21, 2005




Clean Air Two-Stroke for Utility Engine Applications


CARB Grant Number 94-351


The statements and conclusions in this Report are those of the grantee and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Scope and Purpose
BKM has developed and demonstrated a prototype single cylinder engine based on a novel Electronic Direct Fue l Injection (EDFI) system tailored to small, low cost and high production volume two -stroke engines. By offering non-exclusive license options to several engine builders as well as attracting some public funding through the California Air Resources Board (CARB), BKM formed a funding consortium to develop and demonstrate this system. The recipients of this report are the consortium members who assisted with the funding and who have secured non- exclusive technology license options. We have demonstrated exhaust emissions compliance with CARB tier II regulations for the year 2000 and beyond for handheld utility engines. We have also completed preliminary testing on a 50cc moped installation. Suzuki Corporation in Japan is currently conduction additional testing on this 50cc engine. In another program, our license option holder in China, Honglin, is currently operating the system on a 125cc Nanfang motorcycle for demonstration to engine manufacturers within their country. Five samples of this 125cc motorcycle have been manufactured. Photographs of this accomplishment are included in Appendix A.
This report will provide license and license option holders who participated in the consortium program with detailed results of the design and development activity. While the contents of this report may be considered as technology transfer material, BKM acknowledges that true technology transfer must involve ongoing communication and cooperation for the benefit of all stakeholders in the technology.
Due to the high power density and simple construction of the two -stroke cycle gasoline engine, it has been instrumental in the development of the two-wheeler transportation market, the outboard marine engine market and the handheld power equipment industry. However, the exhaust emissions from conventional two-stroke engines are very high due to the basic design and operating principles of the engine. These engines produce from 10 to 15 times the levels of unburned hydrocarbons compared to four-cycle engines.
In a conventional, carbureted two -stroke engine, the fuel air mixture is pumped into the cylinder during a portion of the cycle in which both the intake and exhaust ports are open. The primary activity during this portion of the engine cycle is the scavenging, or removal of combustion byproducts from the previous engine cycle. This process results in the loss of approximately 30% of the fuel, which escapes out the exhaust port prior to ignition. This loss of both fuel and fresh air is referred to as "scavenge loss". Figure 1 illustrates the scavenge loss of a contemporary two-stroke utility engine.
Figure 1. Two-stroke Engine Scavenging Loss
The high level of exhaust emissions and poor fuel economy typical of small piston ported two-stroke spark ignited engines mandates the need for improved combustion over the operating range of the engine. Direct, in-cylinder injection has been demonstrated to significantly reduce unburned hydrocarbon emissions by timing the injection of fuel in such a way as to prevent the escape of unburned fuel from the exhaust port during the scavenging process.
Figure 2 illustrates the typical relationship between exhaust emissions and the air/fuel ratio, defined by the excess air factor lambda. Lambda is the ratio between actual air/fuel ratio and stoichiometric air/fuel ratio. Stoichiometric air/fuel ratio is the theoretically perfect ratio for most efficient and complete burning. Lambda less than 1.0 is a rich mixture and lambda greater than 1.0 is a lean mixture.

In a naturally aspirated engine such as the low cost two -stroke, air supply is dependent on the piston motion and engine power is proportional to the amount of fuel burned. Therefore, a rich mixture increases power and a lean mixture reduces power.
As shown in Figure 2, many contemporary two -stroke engines operate in the range of 0.70 to 0.75 lambda in order to optimize power and reduce combustion temperature. Unfortunately, this condition results in very high CO emissions as well as adding to the already high unburned HC emissions. The Oxides of Nitrogen (NOx) emissions however, are very low due to the low temperature of this rich combustion mixture.
Figure 2. Influence of Excess Air Factor, Lambda, on Emissions
Exhaust emissions can be minimized if lambda is very lean (greater than approximately 1.5). Such lean air/fuel ratios may be achievable using direct injection of fuel as proposed. However, without additional air charge boosting, maximum engine power is reduced to an unacceptable level. In the range of lambda 0.85 to 0.95, emissions can be minimized without significant power loss. It has been demonstrated that the combination of in-cylinder fuel injection (reduced scavenge loss) and operation in this air/fuel ratio range (=0.85-0.95) results in significantly reduced emissions levels.
Compounding the basic two-stroke inefficiencies described above, it is normal for crankcase scavenged two-stroke engines to misfire at part load. Part load operation of spark ignited engines involves reducing both the fuel flow and throttling the airflow through the engine in an attempt to maintain an ignitable, stoichiometric air/fuel mixture. Misfire at part load in a two-stroke engine is caused by the presence of residual exhaust gas, degraded scavenge efficiency and the resulting degraded air/fuel ratio control. This part load misfire contributes greatly to added unburned fuel emissions and increased fuel consumption. Direct in-cylinder injection alone does not solve this part load misfire problem.
The dynamic fueling range is another challenge for fuel injection equipment. The fuel injector must accommodate both the full load fueling rate, as well as the minimum fueling rate required to idle the engine. A major difficulty with conventional fuel injection concepts for small two-stroke engines is the inability to provide precise well-atomized fuel sprays at these very small fuel deliveries, particularly as fuel consumption and emissions are reduced.


Funding Source Funding Amount

ICAT $199,491
Grantee $999,330


Click here for the entire final report.
Click here for technology brochure.


ICAT Funded Projects Page