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Small Spark-Ignited (SI) Engines: Catalyst Applications

November 29, 2013

At Hypercat ACP, we loosely categorize small spark-ignited (SI) engines as those being 25 hp, or less.  Examples of such engines include, but are not limited to those found in miscellaneous lawn equipment, hand-held devices, outdoor generators, snow-blowers, certain marine applications, and motorized scooters.  The US Environmental Protection Agency (USEPA) formally classifies “Small SI Engines” as follows:

• Nonroad – Primarily used in lawn & garden applications

  • Power ratings ≤ 19 kW

• Non-Handheld

  • Class I = >80 cc and <225 cc [engine displacement]
  • Class II = ≥225 cc

• Handheld – Carried during operation

  • Class III = <20 cc
  • Class IV = ≥20 cc and <50 cc
  • Class V = ≥50 cc


As for an appreciation of the scale of the number of Small SI Engines available throughout the world, according to USEPA statistics, for Model Year (MY) 2011, total production in the US alone was an estimated 27 million engines.  Furthermore, for MY 2011, this quantity represented approximately 25% of the total number of comparably classified engines produced worldwide for that particular Model Year.


Contrary to the belief of some, USEPA regulations do exist for Small SI Engines.  Phase 1 regulations became effective with Model Year 1997.  Phase 2 regulations were phased in between MY 2001 and MY 2007.  Phase 3 regulations began in 2009, and were fully implemented in MY 2012.  Phase 1 & 2 were applicable only to the exhaust, while Phase 3 regulations included both exhaust and fuel evaporative guidance.  Phase 3 Exhaust Standards are listed below:


Small SI Engine USEPA Phase 3 Exhaust Standards

Engine Class

HC + NOx (g/kW-hr)

CO (g/kW-hr)

Effective Date

Class I



MY 2012

Class II



MY 2011

Class III



MY 2010

Class IV



MY 2010

Class V



MY 2010


Here at Hypercat ACP, we manufacture Three-Way catalysts that meet emissions requirements for Hydrocarbon (HC), Carbon Monoxide (CO) and Nitrogen Oxide (NOx) for Small SI Engines and conform to the high expectations of our customers.

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