This idea may not be the most successful solution to improve fuel consumption but I can build the concept almost for fre "Scrapheap Challenge-style". I have been practising Pulse & Glide (accelerating/coasting) together with engine shutdown in city traffic, saving some fuel. About 50% of my travel TIME is in city traffic with a lot of traffic lights, the rest is highway where I go maximum 80km/h (50mph).
My idea is to use a tiny diesel engine, a 12hp@3200 rpm 600cc two-cylinder industrial Kubota in two ways, -as a "coasting booster" that hopefully will extend the time between accelerations, but also as the main engine for slow city traffic. A larger diesel, preferably a small turbo diesel, as the only engine would offcourse be a much better solution but I have to satisfy with what I have in my garage. A great bonus with an extra power unit is that I don't have to worry about expensive towing if my old scrappy Saab 96 V4 breaks down on the highway.
The concept:
The little Kubota will (almost) fit in the trunk together with the hydrostatic transmisson I got together with the engine. Hydrostatics have poor efficiency (85-90% in my case) but that may not hurt too bad in city traffic where a lot of the time is spent idling at the traffic lights. Together with a weak engine the continous variable transmission may also be superior to a stepped gearbox. Since the fuel consumption at idle is extremely low on such a tiny diesel I don't need to bother about shutting down the engine at the traffic lights. The diesel will have an alternator big enough to run LED rear- & headlights, heating fan and rear window defroster. There will also be an electric coolant pump and the diesel will share radiator with the Saab's petrol engine. This will not only save weight but also keep the big engine warm until next time it's needed.
The output from the diesel will be through a 5:th wheel out through the rear of the car. It will be a small 10" or 12" wheel with a pneumatic, electric or hydraulic cylinder to lift the wheel when it's not needed. This design may look a bit silly but a major advantage is that I can design the entire diesel unit as "cargo" instead of having to pay a lot of money and effort to get it legally approved as a modification to the car.
I expect to get a maximum speed of about 40-50km/h from the diesel and hydrostatic transmission, but the 12 horsepowers is also about half or two thirds of what's needed to KEEP my highway cruising speed of 80km/h (50mph). Att highway I simply bypass the hydrostatic transmission with a direct gear between the engine and the 5:th wheel to get highest possible efficiency.
The Saab's petrol engine will offcourse be needed for acceleration from 50 to 80 but then I can hopefully shut it down a lot longer than without the help from the little diesel.
To make the design as simple as possible there may actually be two little wheels sticking out from the trunk, -one for the highway direct drive (via a V-belt), one from the hydrostatic transmission for city driving. The highway wheel can be spring-loaded to the ground continously since it will be disengaged by slacking off the V-belt, but the city-wheel must be possible to quickly lift up from the driving seat.
12hp@3200 rmp is a low figure also for a 600cc engine. Perhaps it can easily be adjusted to give a bit more since it's an industrial engine? I suppose the same or similar engines may have been made for other purposes such as boats where a bit more wear can be tolerated in exchange for more power.