For a while now, I’ve been planning a tadpole (also known as reverse trike) three-wheel vehicle that would have tandem seating, an enclosed cabin, and would achieve 156+ miles per US gallon on the highway by using a diesel engine.
I’ve even bought a 654 cc twin-cylinder Ruggerini diesel engine for this project. Here’s the problem though.
That diesel will get me in the ballpark of 16-20 hp. While that will be more than enough to achieve cruising speed, 16 hp won’t get me acceptable acceleration, and even 20 hp will be painful. In addition, the driveline will be rather long, due to having the transmission behind the engine (and a fair distance away to avoid fouling the oil filter), and then the rear wheel behind that.
So, my question to you is… should I abandon the diesel route, and go to a 300 cc single-cylinder fuel injected gasoline engine from a scooter (which means CVT)?
The major downsides would be a loss of bottom-end torque, a major loss in efficiency – it would likely struggle to get much past 100 miles per gallon – and a loss of ability to run sustainable biofuels.
On the flipside, vehicle weight (which is not everything when it comes to fuel efficiency – aerodynamics is far more important – but it helps, and it also improves handling and stability, especially because all of the weight is coming out of the rear) is reduced significantly, vehicle length is reduced significantly (making parking lot maneuvering significantly better, especially given that the first prototype will have no reverse gear), assembly is greatly simplified (I don’t really need to worry about the rear section of the vehicle at all, as it can come from the donor scooter, engine cooling is improved (water instead of air, which also means a reasonable way to get cabin heat), and overall cost might be reduced. Oh, and I get 29.5 hp @ 8500 instead of 16 @ 3600 or maybe 20 @ 4000, albeit only 18 ft-lbs @ 6500 instead of 23.6 @ 2400.
Another factor is, if the prototype is successful, I’d like to put it into production, and as much as I hate to admit it, a CVT will be seen as more desirable than a manual for much of the potential target market. And, the cheaper factor helps, too, given that I have a (admittedly pie-in-the-skyish goal) of selling these things for $5000 for a base model. However, a reverse gear may be harder to implement with such a setup…
So, is it worth the loss in economy to get all these benefits? You be the judge, let me know in the comments here or wherever you saw this post, thanks!