Cummins leads the way with advances in Brake Thermal Efficiency

[HoleshotHolset]

Interesting reading...

http://www.greencarcongress.com/2005/10/cummins_targeti.html



Matt

 

[wxman]

Thanks for the link.



That appears to be a somewhat different approach than what is being proposed by Caterpillar for their 50% BTE 2010 compliant HD engine.



http://www.osti.gov/fcvt/deer2005/Easley.pdf



Something I find interesting is that Cat suggests that the optimum compression ration is 22:1 (page 22) and the optimum boost pressure is ~50 psi (page 23 - 330 kPa = 3. 3 atmospheres = ~50psi assuming my conversions are correct). However, the diesel engine manufacturers seem to be lowering CR as the boost pressure is raised, at least for our light heavy-duty engines. Anyone know why?

 

[RustyJC]

However, the diesel engine manufacturers seem to be lowering CR as the boost pressure is raised, at least for our light heavy-duty engines. Anyone know why?

In order to limit peak firing pressures and the resulting stresses on the engine components.



Rusty

 

[Alan Reagan]

I believe that the Cummins engineers lean toward the CR of 19:1 as optimum. There are diminishing returns with going higher and thinking it is better. The higher the ratio, the harder the engine is having to work to compress. And the combustion/burn efficiency doesn't necessarily increase with the higher CR. It just creates more heat.

 

[RustyJC]

There's really nothing magic about any given compression ratio. A key factor in determining an engine's design compression ratio is the surface area-to-volume ratio of the combustion region and the associated heat loss during compression. As engines get larger, not as much compression ratio is required to bring the temperature in the combustion chamber up to the ignition point of #2 diesel. For example, the Cooper-Bessemer LSVB diesel engine (a 4-cycle, turbocharged and intercooled industrial diesel) has a 15. 5" bore and a 22" stroke - the compression ratio is 11:1.



Rusty

 

[cojhl2]

The CR by itself is not the most important issue. The reason for any compression at all is to concentrate the oxygen enough to get a quick burn of the fuel. The concentration is obtained by the volume of air in the cylinder at the start of compression and the CR.



Air under pressure (power or normal aspirated) must be enough to concentrate the oxygen and raise the temp enough for combustion.



Thats the way I see it.

 

[wxman]

I was under the impression that one reason (main reason?) why the CI diesel cycle is more efficient than the SI otto cycle is because of the much higher CR that can be achieved in the diesel cycle. I understand that boost pressure has an affect on the "effective" CR, but it seems that the trend to lower CR is not a good thing from an efficiency perspective.



Can anyone shed additional light on this?

 

[HoleshotHolset]

I understand that boost pressure has an affect on the "effective" CR, but it seems that the trend to lower CR is not a good thing from an efficiency perspective.

I think you answered your suspicions in that one sentence...

The goal is to make the engine efficient once it is up on boost. Sure, you may sacrifice a little efficiency when idling, but the vast majority of the engine's life is spent under boost.



Matt