Engine Efficiency and Thermodynamic Cycles

A comment on my post announcing the first firing of the CCI has inspired me to write a bit about what kind of inherent limitations different engines have and how the CCI’s theoretical limitations are higher.  I will spare you the Pressure-Volume diagrams we love to look at here at Motiv.  Instead, we’ll go straight to the numbers computed from those diagrams.

The engine Americans are most familiar with is the gasoline automobile engine.  This engine uses what is called the Otto Cycle.  Then there are diesel engines which predictably use the Diesel Cycle.  Additional cycles are Atkinson, which is similar to Otto but expanding exhaust all the way to atmospheric pressure before sending it out the pipe, and Brayton, the cycle used by jet engines and our CCI, which has the expansion of the Atkinson Cycle, but the constant pressure combustion of the Diesel.  Finally, there is the Carnot Cycle, the cycle that theoretically is the most efficient but engineers have struggled to implement this cycle in a real engine that is affordable, practical to use and is able to maintain the high potential efficiency.  If we try to compare these cycles against each other, it is useful to start out doing this at the same peak pressure ratio so we can see the effect on efficiency of cycle only.  In the real world, the differences will be even more dramatic because the Otto and Diesel cycles generally must use much lower pressure ratios for practical reasons.  In this chart, we compare all cycles at the high pressure ratio used by the CCI Engine.

The efficiencies in this table are theoretical maximums, assuming the engines are all frictionless, have no heat loss, 100% reversible combustion and do not have any accessories like oil pumps, alternators, etc.  All of these things reduce efficiency.  You can see that an engine using the Brayton Cycle has an inherent advantage over gasoline and diesel engines that have a much lower theoretical limit.  Consider, however, that there are practical limits to the compression ratios that a diesel can use which would further limit it to around 56%.  The Otto Cycle has even greater constraints on compression ratio that limit it’s theoretical efficiency even more.  A note about jet engines using the Brayton Cycle: their turbines are not cabable of generating the same kind of pressure ratios that the pistons of the CCI can, which limits their efficiency compared to the CCI.

There are a lot of new engines out there such as Achates Power, EcoMotors and Pinnacle Engines that are working on alternative architecture engines claiming higher efficiency, but they are all working off the same old LIMITED cycles.  This makes their claims on significantly higher efficiency hard to believe because thermodynamically these engines are not different from ones that have been built millions of times before.  Some of them base claims of higher “efficiency” in a car by shutting down some cylinders during highway use or combining them in hybrid systems.  I find these claims to be very misleading because it has nothing to do with the efficiency of the engine and is a strategy that can be used by ANY engine to reduce the fuel consumption of a vehicle.  Then there is the Michigan State University Wave Disk Engine that has claimed a 400% to 500% improvement in efficiency.  A professor of mechanical engineering at a prestigious university using such absurd and obviously false hyperbole to obtain an ARPA-E grant makes me cringe.

I try to always be accurate and objective when talking about the CCI engine and perhaps that gets lost in the weeds of the stretched claims by many of my competitors.  Thankfully, I think even against many of the unrealistic stats they publish, our realistic ones still look a lot more attractive!