I won't deny, old-school Diesels are still my favorite engines for many reasons, ranging from their low electromagnetic emissions (a particularly-critical point in military operations) and adaptability to alternative fuels, but the spark-ignited engines still have some valid points, mostly regarding a lower initial purchase cost, smoothier operation and improved acoustic comfort, but also considering an easier cold-start. Another advantage is the ability to run on gaseous fuels such as LPG and the increasing popular CNG/LNG without the need of a pilot-injection of the main liquid fuel, or to run on ethanol without any ignition-improver additive.
Well, very few attempts were done on that matter, mostly adapting a 4-stroke spark-ignited engine (Otto cycle) to run on compression-ignition at higher RPMs, and actually didn't hit the market. GM did some tests in Europe with a modified version of the 2.2L Ecotec engine into some Opel Insignia mules, and Mercedes-Benz once featured a concept-car, the F700, with an engine labelled as DiesOtto, which had the very same operating principle. Both had a 4-cylinder engine fitted with direct-injection, with the spark-ignition retained to decrease vibration and harshness at lower RPMs and switching to some sort of homogeneous-charge compression ignition (HCCI, developed by the Australian company Orbital Engines) at higher engine loads and constant speed.
Nowadays, with Mazda applying the same 14:1 compression ratio at its entire SkyActive engines range (unless in Australia, Africa, Middle East and Americas where a 13.5:1 ratio is adopted in the 2.0L and 2.5L gasoline-powered and gasoline/ethanol "flexfuel" versions) including not just the Diesel-powered SkyActiv-D but also the Sky-Activ-G spark-ignited ones, it seems like one step closer to a commercial viability of the HCCI concept. Eventually, the major issue would make it compliant to stringent emission standards while still keeping the multi-fuel ability at its best.
The most commercially-successful initiative at this field are still the Evinrude MFE (Multi-Fuel Engine) outboard motors, still only available in 30-hp (E30MRL) and 55-hp (E55MRL with a regular propeller and E55MJRL with jet-pump propulsion) ratings, and only for the U.S. Department of Defense, certified to use regular gasoline, JP-4, JP-5, JP-8, Jet-A and Jet-B as primary fuels, and regular Diesel fuel in an exceptional/emergency situation. These engines are 2-stroke instead of 4-stroke, but also use a direct-injection system developed by Orbital Engines, basically the same as the E-TEC featured on its civilian products, and keep the spark-ignition at its entire RPM band. A good advantage from keeping the spark-ignition matched to the 2-stroke cycle is that it doesn't require a supercharger to provide scavenging pressure as a Diesel 2-stroke does.
At a certain extent, the multi-fuel ability provided by the Evinrude military-spec outboards can even relate to the ancient "hot-bulb" engines which were widely used in agricultural machinery, railway, marine and industrial/stationary applications before Diesels got mainstream, altough those required pre-heating of the cylinder head with a lamp (usually kerosene-fed) and had the ignition provided by the contact of the fuel with the heated internal surface of a hot bulb mounted at the top of the head. It's worth to note that spark-ignited engines usually have a broader RPM band when compared to Diesels which internal components are usually heavier to stand to the higher compression ratioes, while the ancient "hot-bulb" system were even more limited on that matter mostly due to thermal management and ignition timing issues.
Electromagnetic radiation either from an electric ignition or an electronic fuel injection has been a matter of concern for military fleets since it eases the detection for enemy radars, no wonder mechanically-governed Diesel engines are still available for such operations even in markets with strict emission standards turning them unsuitable for newer civilian vehicles, but actually even military fleets are already turning to the EFI mostly due to the ease and quickness of an on-board diagnostics platform (currently OBD-2 is standard) for troubleshooting at the technical support locations.
Sure the milder emission requirements for military fleets is easier to overcome, but there are also good perspectives for this system in civilian applications, ranging from stationary/industrial, recreational marine, commercial transport, off-road vehicles for extreme environmental conditions (such as a snowmobile) or even in a light-duty road vehicle. One can easily guess Evinrude would have a great marketing opportunity if they would make MFE engines available for the average Joe, even if they would ditch the tactical-black paint (rednecks would rather want it camo anyway) and fit it with an electric starter as a standard feature while the military versions retain a good old-school manual cord-pulling recoil starter for enhanced reliability.
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I have seen one of those Evinrude motors, a neighbor of my parents bought one and he's not a military.
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