No-Smoke Two-Stroke


Two-Stroke Research


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Figure 1:  Percent short circuited fuel for the Arctic    Cat 600 EFI
Although two-stroke engines have simple mechanical operation and nearly twice the power per displacement as four-stoke engines, they are losing grasp of the recreational snowmobile market.  The two-stroke engine has excessive exhaust emissions, noise emissions, poor fuel economy, and poor low load operation.  If the two-stroke engine is not cleaned up it will be banned from operation; they are already on their way to being banned from national parks.  This is because of the greater restrictions being proposed by the National Parks Service (NPS). The NPS is requiring that recreational snowmobiles entering Yellowstone National Park meet Best Available Technology (BAT) standards for emissions.  The emissions must be less than 15 g/kW-hr for hydrocarbon (HC) and 120 g/kW-hr for carbon monoxide (CO) (EPA, 2003).  Four-stroke engines easily meet these standards, whereas two-strokes do not.  The results from experiments conducted at Southwest Research Institute, Table 1, clearly indicate the difference in exhaust emissions between two and four-stroke engines.  Table 2 shows the results of fuel economy experiments conducted by the 2003 UICSC team.  The 2003 competition snowmobile and the Arctic Cat were tested on the same day to ensure the same duty cycle was used so a direct compression of fuel economy could be made.    The difference in exhaust emissions and fuel economy is evident.  If two-stroke engines are going to continue offer low weight and high power output for snowmobiles, they Text Box: Snowmobile Engine	HC g/hp-hr	CO g/hp-hr	NOx g/hp-hr	BSFC lb/hp-hr
4-Stroke Mean	3.50	59.3	6.57	0.65
2-Stroke Mean	140.7	385.1	0.54	1.08
Table 1: 2-Stroke and 4-Stroke Five-mode engine brake specific emissions and fuel consumption running on 10% ethanol fuel (Lela and White, 2002)

S    Snowmobile	Fuel Efficiency (miles/gallon)
2002 U of I Competition 749cc 4-Stroke	16.0
2003 U of I Competition 833cc 4-stroke	18.3
2003 Arctic-Cat Mountain Cat 600cc EFI	7.5
Table 2:  2-Stroke and 4-Stroke snowmobile fuel efficiency (Bradbury, French, 2003)

must be cleaned up and the fuel efficiency improved.

The very thing that makes two-stroke engines mechanically simple causes them have poor fuel economy and high exhaust emissions.  Unlike a four-stroke that has four distinct cycles- intake, compression, power and exhaust- the two-stroke engine does all of these operations in just two cycles. For an insight into this operation, visit


There are two very undesirable side effects of the two-stroke cycle, the mixing of the fresh air/fuel mixture with exhaust gas residuals and the short-circuiting of the fresh charge.  Tests performed at the University of Idaho show that as much as 50% of the fresh fuel charge can be short-circuited; see Figure 1.  The range of throttle position and rpm that matches the 50% short-circuited fuel is an operating zone that never actually happens.  Normal two-stroke engine operating ranges see short-circuited fuel ranging between 35% and 20%.


The largest amount of short-circuited fuel occurs at light-load, low engine rpm operation (Ramakrishnan, 2001).  This is due to bad combustion and/or misfire at light load (Gentili, et al, 2001).  The poor combustion and misfire is attributed to air-intake throttling.  The restriction on the intake side of the scavenging reduces the scavenging ratio and leaves excessive residual exhaust gasses in the cylinder.  The large amounts of exhaust gasses present in the combustion chamber lead to misfire and high emissions.