As for safety, the hydrogen won't detonate because there isn't an oxidizer, but if air should be accidentally introduced to the system, the motor will just start up and run, diesel motors can handle explosion from various fuels such as ether, ethonal, diesel, gasoline, propane, etc, etc, and oxidizers such as pressurized air and nitrous oxide. If the mixture explodes in the intake or exhaust manifold instead of the cylinder,... well just how hard is it to make sure air doesn't enter the system?
I want to add some more information to my idea. After the solvent is injected into the cylinder and the Piston goes down, the solvent hydrogen mixture will then be pushed into the exhaust manifold. Now remember, the injectors turn this solvent into a really fine mist that acts like a gas, as this mist goes onto the exhaust manifold some of it will turn back to liquid that will collect in a low place between the exhaust and intake manifold (remember the two manifolds are connected so it can continuesly loop). Now the liquid is pumped from the manifold back to the injectors , where it can be injected again into the hydrogen. So the hydrogen goes in a loop from the cylinder, to the exhaust manifold, then to the intake manifold, them back to the cylinder. Then just keep turning the motor for as long as necessary.
It doesn't need to be a diesel motor, a gasoline motor can be used, with the solvent injected into the intake manifold instead of direct injection into the cylinder.
Heat is a major issue because as it's compressed to a 20 to 1 ratio it will get extremely hot. To answer this problem, cool the hydrogen with dry ice before it enters the cylinder. Also pack dry ice around the motor's radiator to get the coolent very cold, and use alcohol in the motor's coolent system