Spark Ignition Diesel - a better engine ?

[AppRenticE]

I've always liked diesel motorcycles especially the simple single barrel ones, they have a charm of their own mostly at cruise speeds. But at the same time i never have had the guts to own one for they scare me off with heavy vibes at idle & slow acceleration response. Is there a way to give the engine some manners? to make our rides easier.

To my understanding (limited though) it is the high compression ratio primarily causing this nature. My search for low compression diesels revealed some SAE papers but no build attempt on the same lines.

http://subscriptions.sae.org/content/920545/

In the present work, investigations were carried out on a single cylinder, low compression ratio, spark-assisted low heat rejection D.I diesel engine. An extended electrode spark plug was used. Performance and emission tests on the engine were carried out with diesel fuel at two compression ratios, 10.5 and 12.5. In each case the engine was tested as a normal engine as well as a low heat rejection engine. The test results show that the low compression ratio spark assisted diesel engine operates very smoothly due to the low peak pressure and low rate of pressure rise. The low heat rejection spark assisted diesel engine gave an improved performance and reduced emissions compared to the normal baseline diesel engine. The low heat rejection, spark assisted diesel engine at 12.5 compression ratio gave the best performance among the tested configurations, with an improved thermal efficiency of 3% and a reduced exhaust smoke level of 1.5 Bosch units when compared with normal baseline diesel engine with 16.5 compression ratio.

http://papers.sae.org/830588

A single-cylinder, open-chamber direct-injection (OCDI) diesel engine was converted to low compression ratio, spark-assisted operation. A modular construction cylinder head was built for the test work. The research work indicated that on a typical OCDI diesel engine, several spark plug locations are possible to produce successful ignition of a wide range of fuels. Performance tests were run with different compression and swirl ratio combinations. The best combination was found to be 12.2 compression ratio and 10 swirl ratio. The spark-assisted engine (CR 16:1) was performance tested with methanol and DF-2 plus 20% methanol emulsified fuel. The spark was always required with methanol, however, with emulsified fuel spark was desirable for starting and warming up periods.The investigation suggested the feasibility of economically developing multi-fuel spark-assisted diesel engines.

http://books.google.com/books/about/Spa ... icNwAACAAJ

Conventional diesel engines suffer from problems of excessive weight and size and excessively high rates of pressure rise and peak pressure because of the requirement of very high-compression ratios for the purpose of ignition. They also lack multi fuel capability. The use of electrical or other precise ignition means obviates the dependence on high compression for ignition and would allow the diesel engine to operate at the more efficient and practical compression ratio of 12 to 1. To accomplish this, an innovative ignition system with an unusually high rate of energy delivery was used in conjunction with a modified diesel engine. This new design was found to provide approximately equal efficiency under most operating conditions other than high load, and to provide instant cold start at the more desirable compression ratio of between 11 to 12 to 1. However, problems of spark plug fouling by the fuel spray made this approach impractical for this specific application. Approaches for handling the problem of plug fouling have been suggested in this report.

These make me want to be convinced of the feasibility of LCR-SI-DI diesel but not so about its suitability for motorcycles. In theory if atomizing (by injection), vaporizing (by glowplug till engine is warm) & ignition with timing (by spark) are taken care of then diesel should burn good.

What all technical & performance issues might need to be taken care of for motorcycle usage in case of attempting such a build based on any 1cyl genset engine.

Excuse my poor English, its a second language for me. Cheers..

[XLerate]

Well excuse me, but your 'poor English' happens to be better than about 75% of American's errant babblings in this day!

I think much of their complaint there about weight & size is no longer an issue. One reason gasser engines are lighter is because they aren't designed to last longer than 100,000 miles. Anything more is a bonus. Smaller engines aren't even be up to that good on reliability. If they were built to last, like a diesel, the weight would be much closer. Also many smaller diesels have really gone on a diet with lighter castings & use of alloys.

There doesn't seem to be near enough performance improvement, if any at all, to justify adding all the complex parts & system to make spark ignition. More parts to fail, at the worst time. Higher initial and ongoing costs.

Research & Development test engines always perform much better than mass produced that eventually come off assembly lines. Test engines may be optimized & perfected at every single component, not possible on production engines. So, it's promises, promises that don't match reality.

A normal production diesel can be made to perform if blueprinted. Align-boring crank, cam bores, lifter bores & cylinder bores to blueprints, all balanced & clearanced components to precise original specs [or better], block parallel decked & trued [machined to specs & trued relative to crank bore], heads trued, combustion chambers optimized, with decent head porting & performance oriented exhaust, plus a proper turbo, tweaked injection pump & injectors will be one awesome piece of machinery! Twice the original power or very much more is possible.

[tappy]

The use of a lower compression ratio in a diesel reduces the efficiency of the thermodynamic cycle it operates on. So whilst it's possible to get a more civilised, lighter engine running on diesel, it won't be as efficient.

[coachgeo]

The use of a lower compression ratio in a diesel reduces the efficiency of the thermodynamic cycle it operates on. So whilst it's possible to get a more civilised, lighter engine running on diesel, it won't be as efficient.

If I read it right their research proved you wrong. Though the efficency improvement was margional at best.

Think it worked out ok cause along with the lowered compression they upped the heat with a spark/GP plug along with improved autimation of the fuel

[AppRenticE]

hi XLerate, that my language is ok enough though i was hinting failure to convey the crux matter most times.

Now to the topic, I agree that many of current diesels being light & reliable etc. But the single cylinder ones are much crude for motorcycles, partly owing to their industrial lineage. The intended purpose of adding Spark ignition is to facilitate a lower compression ratio (~12:1) & thereby lower combustion pressure characteristics to a refined + comfortable level for motorcycles. All the works i quoted suggest such a low CR engine runs good & performs equally so compared to the original diesel, though may be at a slightly lesser fuel efficiency. Why this tech did not evolve beyond studies? simply there were no takers to justify creating new one from scratch. For industrial & automotive use they have options to deal vibes, noise, etc.

The way i see, trade some full efficiency for a comfortable single pot diesel. For such an altered engine there could be benefits like better acceleration response, lower vibes, lower combustion noise, less flywheel mass required, ease to kick start, better clutch & g'box life, no cracking up of frame tubes, welds etc. Adding a spark ignition set up is less complex compared to working on an ECU run diesel engine. True that it might take good effort to get the injection & ignition timings resolved & but its mostly a tuning aspect only.

For the Diy diesel bike enthusiast who now depends on Yanclones & the like, Doesn't this tech offer some hope?

[Stuart]

Reminds me of an old guy who approached me and Sam at the last Uttoxeter show and said he'd once built a hot bulb engine into a bike. Lower compression than a diesel and not so efficient but (with a big smile on his face) he said that it went really well Cant remember engine details being given. He was a gypsy and wanted to run the bike on oil. I guess he got halfway to where we're now at

[XLerate]

Certainly, bold thinking outside of the box has changed the world. Even Albert Einstein just sat around scratching stuff out on paper, then told others about it. Still others believed he might be on to something and put the thoughts into action. The world has never been the same!

I'm all for new technology & experimentation. Hopefully your curiousity will lead you to your own exciting discoveries. Even if it doesn't, hey, just like these old bikes: enjoy the ride!

[AppRenticE]

Interestingly this isn't new tech, Hesselman engines of 1920's used similar tech though the design was meant for reliability & longevity of oil engines. I read elsewhere that they employed very low CR ~6:1 which took a toll on output & emissions.

The recent works i posted run CR ~12 which sort of delivers equally good on performance & fuel efficiency fronts. Moreover low comp is evolving to be the latest diesel tech - http://ae-plus.com/technology/toyota-de ... on-diesels - What they manage with hi-tech Ecu injection we Diy-ers have to make do with Spark ignition.

Any takers here for this tech?

[AppRenticE]

A good read about Mazda's low CR SKYACTIV-D diesel engines here at http://www.greencarcongress.com/2011/08 ... 10804.html & scroll down to subtitle SKYACTIV-D.

Though not of spark ignition type, the low CR is described pretty advantageous that it can form another topic of discussion

[Blunt Eversmoke]

There are some youtube vids showing a Simson 2stroke moped guzzle pure diesel/2stroke oil mixture (after warming up on gas, gas was tapped off, diesel tapped on and the spark plug cable pulled off, whether pre-mixed or self-mixing carb, I dunno); other ppl showed a Husqvarna 2 stroke moped do the same. May have to do with comparatively high compression ratio (11:1 or so for the Simson, 12:1 for Husqvarna, IIRC) and the fact that the resonator charging principle has the near-cylinder section of the exhaust pipe act as some sort of hot bulb, accumulating some exhaust heat and giving it off to the "reso" part of the fresh charge. Gotta ask the guys doing that how the diesel MPG is, tho'.

[John]

The use of a lower compression ratio in a diesel reduces the efficiency of the thermodynamic cycle it operates on. So whilst it's possible to get a more civilised, lighter engine running on diesel, it won't be as efficient.

If I read it right their research proved you wrong. Though the efficency improvement was margional at best.

I agree. If I understood correctly, there seemed to be a marginal improvement in efficiency at light load, probably due simply to decreased pumping losses with the lower compression ratio. However, at high loads, the spark ignition diesel was LESS efficient than a standard diesel. Perhaps this was because at peak torque requirements, a diesel engines fuel / air ratio gets fairly close to stoichiometric ratios at something like 20:1. The high standard diesel compression ratio would give a better expansion ratio for the combusted fuel than an engine with a mere 12:1 ratio, making it more efficient at maximum torque levels at all engine speeds. In real world conditions, engines in bikes & cars rarely get to work at light load for long. Especially 9 hp diesels in bikes! Nice try hybrid advocators, but no cigar. Give me Rudolf's finest every time. If I wanted a 12:1 or 13:1 spark ignited engine, there's no shortage of bike manufacturers who make them, for those who like that sort of thing....

[AppRenticE]

..... So whilst it's possible to get a more civilised, lighter engine running on diesel, ...

This is the sole point i thought to be of interest to guys here but sadly the terms performance & efficiency smoked it off. Seems most are content with the way industrial diesels are constituted & runs. I for one am pretty scared by the poor acceleration response, afraid of being run over though i like it much at cruise speeds hence looking out for options

Automotive purpose designed single cyl diesels perfrom way better than the Taurus or most such conversions. Take for example the Piaggio (read as PEE AaaH gio) 3-wheelers, those are comparatively responsive for a ~450cc single even with added body weight & 3 people inside (read as 'PEE-AAH-gio' for the bloody racket they throws out)

[DieselFly]

Using a CVT will help your acceleration big time as you can stay on peak power all the time. Most people that ride my bike which makes about 15hp are very suprised at how fast it gets up and goes. Now that get up and go starts to fade at 80kph only because it starts to run out of horse power. But the turbo is going to fix that

[Blunt Eversmoke]

Exactly, the turbo. Maybe it would pay off to blow the low CR spark ignition "diesels" for high load conditions?