RAM 1500 Diesel option still alive

[flyingcow]

I know when I took IC engines in school, hmm, over 10 years ago, we did some calcs, at the time, a turbo, direct injection gasoline motor could get near 100% of the maximum thermodynamic efficiency possible, and was limited by some material properties to that. The potential gains were getting slim. You see that in the new Ford V-6, its getting close to the most we are going to get out of a gas motor for thermal efficiency.

Diesels at the time were were only getting to like half of the max possible thermal efficiency. Common rail, multiple pulsed injection etc was just getting going, variable geometry turbos being researched etc. Very little development since the first diesel had taken place. Professor was a firm believer in working to get the diesel efficiency up to at least 60-70%, which would improve mileage dramatically. Not sure where that number is at today in a commercial diesel.

What you described, I think, is what Cummins has been doing.Common rail, variable speed turbo's etc.

 

[Renze]

Diesels at the time were were only getting to like half of the max possible thermal efficiency. Common rail, multiple pulsed injection etc was just getting going, variable geometry turbos being researched etc. Very little development since the first diesel had taken place. Professor was a firm believer in working to get the diesel efficiency up to at least 60-70%, which would improve mileage dramatically. Not sure where that number is at today in a commercial diesel.

When i was in school we had a school trip to Wartsila Diesel, a ship engine builder. A man could crawl into the crankcase through a cylinder, to give you an idea of the size of those.
They achieved an efficiency of around 44% when operated at their design optimised speed: To give a comparison, the tour guide also said ordinary industrial diesels (variable operating rpm) achieve around 38% and automotive engines around 35%. Gas engines around 24%

The engine efficiency is limited by the compression and combustion pressures. We dont want high pressures (which comes with high temperatures) anymore, because high combustion temperatures cause NoX emission.

What your professor claims, can only be achieved on stationary gen sets or big ocean ships where they run a steam compound unit to recover the exhaust gas heat, and use the heat in the coolant to heat housing units. Then you'd achieve a remarkable efficiency.
If you cant carry the weight of an exhaust gas heated steam set and have no use for coolant heat, you simply cannot achieve such an efficiency. Especially not with the NoX regulations of today.

I have a friend who works for a small builder of industrial turbine engines: He said they are getting only 60% of the fuel efficiency of a Diesel when used as a stand-alone unit: But the fun part of the turbine is no coolant heat of less than the boiling point of water (which makes it impossible to recover this energy unless for heating) but lots of exhaust heat: With the exhaust heat they can run a steam compound installation and achieve better total efficiency than a piston engine, because with a piston engine, 30% of the waste heat (coolant) cannot be recovered and converted into mechanical power.

 

[westcliffe01]

Renze, the coolant heat can be recovered, but it requires a heat pump which basically reverses the refrigeration cycle. The problem with steam is the hazard. Putting such a system into a vehicle which for sure is going to crash and then not killing anyone is a high risk game no automaker will take.

Turbines have a low mechanical efficiency since the turbine blades operate on aerodynamic principles, whereas a piston follows Newtons law almost perfectly. The big advantage of a turbine is that it is a quasi "external combustion" engine so by using a recuperator to extract heat from the exhaust one can reduce fuel flow substantially. To get way up there in efficiency though, there is no getting around the heat pump. The exhaust temperature can then be so cool that it won't burn your hand.

Heat exchangers work best on engines running on natural gas since it is such a clean burning fuel.. Dirty fuel will always create corrosive condensates which then require heat exchangers to be made of exotic steel which finally pushes the price out of the realm of feasibility.

 

[Renze]

Turbines have a low mechanical efficiency since the turbine blades operate on aerodynamic principles, whereas a piston follows Newtons law almost perfectly.

interesting point... I know the fact, but the principle behind it, never occurred to me.

To get way up there in efficiency though, there is no getting around the heat pump. The exhaust temperature can then be so cool that it won't burn your hand.

I dont see auto makers putting a Stirling engine (heat pump) or exhaust gas steam compound into cars... Not just because of crash safety, just because of weight and cost.
Scania trucks of Sweden, did however sell turbocompound in the late 80's...

 

[Lt CHEG]

I know when I took IC engines in school, hmm, over 10 years ago, we did some calcs, at the time, a turbo, direct injection gasoline motor could get near 100% of the maximum thermodynamic efficiency possible, and was limited by some material properties to that. The potential gains were getting slim. You see that in the new Ford V-6, its getting close to the most we are going to get out of a gas motor for thermal efficiency.

Diesels at the time were were only getting to like half of the max possible thermal efficiency. Common rail, multiple pulsed injection etc was just getting going, variable geometry turbos being researched etc. Very little development since the first diesel had taken place. Professor was a firm believer in working to get the diesel efficiency up to at least 60-70%, which would improve mileage dramatically. Not sure where that number is at today in a commercial diesel.

Going back a little over 10 years to ChemE Thermo and some theoreticals, in a perfect universe the ultimate Carnot Cycle efficiency of a combustion engine would always be theoretically higher than the maximum theoretical Carnot efficiency of a compression engine. The problem is that we don't live in a perfect world and all the real world efficiency killers have meant that diesels would be more efficient in this universe. I think what we've been seeing however is a great deal of advances in technology which has helped us take care of a lot of the inefficiencies of the gasoline engine and we've reaped huge increases in economy because of it. This is likely also a big reason why we're seeing the gap in fuel economy between modern diesels and modern gassers shrinking.