ruff numbers ... fuel consumption 5 litres (about 4 kg) per 100 km =40 g/100 km (this guess is based upon what my street-ridden motorcycles use), plus the air mass flow at stoich 15 times higher than the fuel mass flow, total mass flow 640 g/100 km. That is allowing for almost 5% CO in the exhaust!
Almost *anything* will meet that. A half-decently-jetted carbureted petrol engine with no catalyst should do a little better than that, though there might not be much wiggle room. Carbureted engines with no emission controls are usually around 2% - 4% CO, and with careful lean jetting you can get them down to 1%, at least at idle and light load.
My 1990 Yamaha FZR400 is carbureted, has nothing in the way of emission controls, manual cable-operated throttle, manual transmission, doesn't run at stoich. Pre-dates emission standards. Probably horrible ... but probably not far off what your requirements are.
My 2004 Kawasaki ZX10R has open-loop multipoint sequential EFI but no lambda sensor, air-injection into the exhaust ports (basically a reed valve for each cylinder which uses the pulses from the exhaust to pull in air, there is a solenoid-operated valve to enable or disable this system under control of the engine ECU to avoid popping from the exhaust under certain conditions), oxidizing catalyst.
Most motorcycles (probably ATVs as well) are designed for worldwide emission standards - not the outdated North American ones, and they mostly make them all the same. Euro 5+ compliance is a WHOLE different ball game. Most motorcycles nowadays:
- Drive-by-wire throttle, though this isn't universal. The premium models all have it. The smaller and simpler models don't.
- Manual transmission (thankfully)
- Multipoint sequential port EFI with each cylinder individually mapped
- Air-injection (as described above) with the system enabled or disabled under ECU control (allows the engine itself to be operated slightly rich while maintaining lambda=1 at the catalyst)
- Three-way catalyst
- Post-catalyst lambda sensor for catalyst monitoring (this has been standard in the auto industry, but for motorcycles it is new with Euro 5+)
- Definitely the fuel injection calibration includes wall-wetting compensation
- Yes, closed-loop lambda=1 for the portion of the speed and load map that the drive cycles use. (So far, nobody seems to care about full-load operation ... "real driving emissions" ... yet)
- Cam timing has been optimised for emissions compliance ... basically, low overlap. A little in-cylinder EGR is preferable to scavenging with fresh mixture as would have historically been typical for high performance engines. This seems to be behind at least some of the trend towards larger displacement engines in order to keep performance the same.
- Ignition and fuel injection calibration has to address catalyst warm-up and light-up, and maintaining catalyst operation (hot) at light engine load while not melting the catalyst down at high engine load. A lot of Euro 5+ motorcycle engines have scandalous fuel consumption for what they are. Seems like emitting a fraction less HC and CO via keeping the catalyst hot is a higher priority than reducing fuel consumption and CO2 emissions.
- Provide for an evap system. Not required everywhere. California certainly requires it.
- Has to be OBDII compliant.
- And about a million other details.
If you only intend to sell in USA then you don't need a lot of this ... but consider future-proofing (and other-jurisdiction-proofing).
