Which engine should i use

[mikejaywilson]

i have a tdr250 yamaha and i am fitting it with a diesel single .
hatz 250 or 350cc or yanmar all rev to about 3600 rpm i need to cruse at 80 mph i dont think the smaller engines will do it.
the smaller engines are quite light and the larger ones vibrate too much.
any suggestions ? open to all.
mike.

[smokyjoe]

If you want to cruise at 80 you'll need about 25 HP+! Quite a heavy Diesel, then you'll need a heavier frame, wheels, and transmission, which will add more weight, requiring more horsepower to get decent acceleration, etc. etc. Check what other successful builders have done on this site and their results. There's some really great examples, and I don't think that you will find more Diesel bike info anywhere else! There was a good discussion a while back on the amount of horsepower you'd need for how fast you want to go.

Good luck and have fun with your build

[Stuart]

Yes, I'll back up what SmokeyJoe says. You'll need 20+ hp at least and importantly you'll need to have something that revs over 3800 or you wont make the 80mph. The Ruggerini MD191 gives 19hp and gets me to 70 flat out.
Stuart

[Anorak_ian]

Hi Mike, my bike has a 23hp engine @3600rpm it gets to 80mph quite easily it then takes a short while to get up to 85mph.
The top speed so far is 87mph (speedo and satnav).
The above was motorway riding with bulky camping gear; it can't do those speeds on winding hilly roads such as the A303.

Sadly I haven’t taken her out without camping gear to find out what she will do, and now I am changing her gearbox for one that is not so wide ratio (due to the bad operation of the box), so the top speed will drop a bit. The maths says 85mph with the new box, so 80mph with camping gear.
Well thought out gear ratios are important if you want a particular speed.

Ian

[John]

Hi Mike. No single cylinder diesel is going to offer the kind of top speed, let alone cruising speed that you seek. The problem is the rated rpm.
Most small industrial diesels develop maximum power at 3600. This being the case, the only parameter that changes, is the amount of torque the engine develops to raise the hp at those revs. Without a turbocharger or supercharger, a naturally aspirated diesel produces much less torque than an equivalent capacity petrol engine, due to pumping losses through the gigantic compression ratio.
To achieve 80 mph at all, would require gearing of 22.22 mph per 1000rpm, & require at least 23 bhp.
A 406-418cc single offers about 8.8 bhp & 50 mph cruising with a top speed of about 57. A 462cc Hatz gives 11 bhp & might see 65 mph.
There is a particular leisurely mindset that goes with diesel motorcycling, where the journey is the pleasure, rather than the destination. It's the joy of relaxing travel, rather than the time you have to get there. To be honest, for a balance of reasonable economy & performance, you might be happier with a petrol Enfield. The modern 27 bhp 500 can see 100 mpg without too much coaxing, & cruise at 65-70, with a top speed of about 80 & a bit.
Given your expectations, & your donor frame capability, I think a diesel engine would make you feel endlessly frustrated. They aren't suited to press on type personalities. John.