In a message dated 3/29/2007 9:22:13 PM Eastern Standard Time, gwbraly@gami.com writes:

I've seen many intercoolers, both diesel and gas. However, you stated that diesels require much larger radiators than spark/ottocycle engines. This just isn't so. I have a 30 hp gas tractor and a 50 hp diesel tractor. Both are naturaly aspirated. The diesel radiator is the same width and thickness but about 20% taller. Yet the diesel makes 40% more power. Granted there is more to the heat transfer capacity of a radiator than its physical dimensions; fin spacing and materials are two examples.

 

Of course a 500 hp diesel engine in an earth mover or tractor trailer rig would have a substantially larger radiator than a 500 hp gas engine in a Viper sports car. But there are two major reasons for this that have nothing to do with the fuel used. On the street, the Viper will have a hard time using 500 hp for more than 20 seconds at a time. Most of its life it will operate at 10% or less of its rated power. The earth mover utilizes well over half of its its rated power most of its working life and may operate at full power for tens of minutes without a break. Asuming around 30% of the fuel energy is lost to the cooling system, it is pretty obvious that the engine operating at the higher power setting will need the bigger radiator.  Also, the penalty for carrying an additional 50 lbs of excess radiator and coolant on an earthmover or semi truck is inconsequential, where as for the Viper this would negate the weight savings of 50 lbs of aluminum used in the car.

 

The principle efficiency advantage of a diesel is found in part throttle operation because there is no throttling loss as compared to a gas/ottocycle engine. At full power operation, there is very little efficiency advantage to be found for the diesel.

 

At full power, the diesel exhaust energy is close to that of the gas engine. Yes, the diesel exhaust is cooler, but this is primarily due to its operation with excess air, which is required for complete combustion and increases the mass flow rate. If the diesel engine is to have the same or slightly better thermal efficiency, the cooling loss cannot be substantially higher than that of the gas engine. Thus, in a simillar application the radiators of a gas and diesel engine should not be dramaticly different. This is just what we see in the real world.

 

Personally, I am not a huge proponant of diesels for aircraft. The big reason is the additional weight of the assembly. On a longer flight, operating at substantially reduced power, the efficiency might offset the weight penealty.  Of course this ignores future fuel availabillity issues. Each application/flight profile needs to be evaluated individually.

 

Rob