50cc Gas to Diesel Engine Conversion

[sendler]

Originally Posted by BobS

bigscoot,
To my mind, the easiest way to get 50cc from something like the popular Lifan 140cc engine is to delay the intake valve closure, which reduces the displacement.

Trying to alter an engine for faux Atkinson cycle with a late intake valve only works with fuel injection. Carbs don't like the back and forth air flow. And they are trying to build a diesel out of a gas engine. Which means doubling the stock compression.

[Varn]

Seems to me that the most difficult part of your project would be to find or manufacture a fuel pump for the required displacement.

[renault_megane_dci]

A diesel engine usually has a bore to stroke ratio very much towards long stroke because the diesel burns slower so is basically useless above 5K rpm so the torque advantage of the longer stroke has to be used.

If building a small capacity diesel, I would seek for this bore to stroke ratio in bigger capacity gas engine and then building a top end with home made head and cylinder built from small capacity engines.
(doing the CR is gonna ask for a very shallow chamber volume hence a flat head sounds a good idea)

[BobS]

Sendler, in post 27, bigscoot said that they would use a mulit-inject system so no carb and, since the Mazda Skyactiv diesel uses the gas engine ratio of 14:1, perhaps no increase in compression increase is needed. However, Mazda uses a glowplug for ignition and Bigscoot also has the sparkplug as an optionignition source
Bigscoot, looking for a bigger engine, I would like to recommend the relatively inexpensive Lifan 110cc engine. It has a stroke of 4.95 cm compared to the Zuma 4.36 cm stroke. I calculate a bore size of 3.535 cm to keep the 50cc displacement and the Lifan starts with a bore of 5.24 cm so a sleeve would have to be .85 cm thick or about 1/3" thick. As sendler notes, this could wellt interfere with the valves.
On the other hand, using the zuma cylinder, piston and head with the Lifan's longer stroke means that you would exceed the 50cc displacement without a (very thin) sleeve.
Well, at least your possibles have been clarified for you. Good luck.

[renault_megane_dci]

One word of caution around Mazda's Skyactiv low comp diesel :
it is a turbo diesel !
edit : so volumetric efficency lost in low comp is restored because it is under boost


If building a 50cc engine, turbo is out because there is no appropriate unit available.

It doesn't mean you can't boost it but not with a turbo. (there you need to check the rules because the next best setup would be an electric blower on a battery)

If it has to stay N/A, then you're back to square one and a 18:1 CR.
In fact your benchmark setup is a VW SDi engine with corrections towards capacity applied.
http://en.wikipedia.org/wiki/SDI_(engine)

edit :
wich means you are in the 1.8 hp region.

also you need a fancy piston with a bowl in it to center combustion in the cylinder.

[cRiPpLe_rOoStEr]

Originally Posted by renault_megane_dci

If building a 50cc engine, turbo is out because there is no appropriate unit available.

It doesn't mean you can't boost it but not with a turbo. (there you need to check the rules because the next best setup would be an electric blower on a battery)

I remember Peugeot made some scooters with a Roots-type supercharger.

[samwichse]

Never mind, results were fake

[Bigom]

Sorry to bring this back. How did it go?

My first surprise is how a senior in any mechanical engineering program knows so little about the principles of combustion engines and fuels used.

What are they teaching these days?

[Joris]

You mean the Lohman 18cc diesel engine. That one was capable of getting 125km/l.