Preheating Diesel before injection?

Question

Significant amount of heat is lost from an engine by various means, so to recoup the energy, is it possible to increase efficiency via heating of diesel fuel prior to injection using waste engine heat in passenger cars similiar to marine diesels? Are there any recent journals or studies in this matter?

Answer 1

Consider how much energy is needed to heat the fuel that is injected each time compared to the energy that can be recovered from the turbo-charger for example. One thing to note is that many of the diesel pumps (style DPA / inline high pressure) will compensate for changes in the density of the fuel according to the temperature, the modern common rail injection systems will compensate for this temperature as well.

I did heat the diesel fuel on one of my cars using the radiator from the automatic version of the car and running the fuel through the oil cooler part (blocking the airflow through that bit), result : helped the running of the engine during the cold months but no appreciable change in fuel consumption was noted.

Edit: noticed that you tagged marine-engineering, so large marine diesels use a very different fuel and that is heated before injection so that the viscosity is reduced as the fuel very viscous when cold.

Answer 2

Everything that gets the mixture in the cylinder hotter makes for a more efficient combustion. That is because the power of the engine depends purely on the pressure exerted on the piston, and the effective pressure is caused by temperature rise, and nothing else. So yes, injecting hot diesel would make for a more efficient combustion, in theory. But it would be insignificant. You can heat it up to just below the boiling point, but you'd still need the energy to evaporate the diesel, which is far more than needed to heat it up. Installations for this purpose will cost more than the profit they'd give you.

There is an alternate solution though; heating up the air. Once the air is trapped inside e.g. a cylinder, you could heat it up, and it would gain pressure. That pressure will add up to any pressure caused by combustion. You would get a more efficient combustion. The problem is that you're dealing with a continuous air flow, which is hardly trappable. If you heat it up without trapping it, it will just expand and flow out of the cylinder.

You'd need a pressure tank in which you can trap bulk charges of air, you can then heat up that trapped air, and feed it to the engine once it's hot. But also those installations would only perhaps be cost effective in large static installations. In cars, downsizing and turbochargers are the way to go for realistic and cost-effective gains in efficiency. That's why you see this more and more today.

For marine applications, heavy oils are preheated to reduce their viscosity, which only then enables them to be used for injection. The heaviest oils are so thick you can nearly walk on them when they're cold.

Answer 3

Heating the diesel will have a stronger effect on how it burns in the cylinder than how much energy it adds to the system by recovering waste heat. Diesels depend upon compression to heat and ignite the fuel adding heated fuel would cause it to vaporize sooner and predetonation would be possible.