High Efficiency Hybrid Cycle (HEHC): Finally Something Fresh for Thermodyanmics

For a long time, I had thought that achieving greater than 50% efficiency for a practical internal combustion engine is impossible.  Of course each engine is tuned to a particular fuel type.  However I was wrong.  There is an engine that can exceed 50% real efficiency (75% ideal) in even very small sizes (40hp or less.)  Not just that, this engine isn't tuned to a particular fuel type, and can use a variety of available fuels.  It's called the High Efficiency Hybrid Cycle or HEHC, concieved and prototyped by the start up company LiquidPiston.  Ladies and gentleman, we have a true discovery in the field of thermodynamics, and a remarkable achievement at that.

The Diesel cycle and engine is known for its efficiency in practice.  They are ubiquitous among the internal combustion engines found in buses, trucks, trains, cruise ships, submarines and backup electric power plants.  Even though on paper the gasoline or Otto cycle, engine is more efficient, gasoline knocks, combusting spontaneously without the spark plug at high compression ratios.  In Diesel engines, knocking is essentially the standard method of combustion, using compression, rather than a spark, to ignite the fuel.  This of course means this problem is inapplicable to them.  Therefore, Diesel engines can be designed in the real world with much higher compression ratios.  The efficiency of internal combustion engines (ICEs) is defined based on the compression ratio; the higher it is, the more efficient the engine.  This has made Diesel engines the most efficient ICE engines up until now.

Left: Diesel Engine, Right: HEHC Engine

While some of the sources claim the LiquidPiston HEHC engine is a diesel engine, it isn't limited to it.  This engine solves two problems.  One is the inherent poor efficiency of the ICE, unchanged for nearly 100 years.  The second is that ICE cycles are tuned to specific fuels.  The HEHC engine runs on a variety of fuels, and is also remarkably efficient because it can achieve compression ratios like a Diesel engine, with better overall efficiency.  Additionally, the engine's power to weight ratio is 5 times better than an equivalent Diesel--about 2 lbs per hp, compared with 10 lbs per hp.  This is important for propulsion applications.

While other cycles and engine designs have these properties, but they aren't useful for automotive and small engine use.  For example, Rankine cycle engines must be very large--like large enough to be contained in a building--mostly useful for driving banks of steam turbines.  Stirling engines, while efficient and flexible with regards to fuel and heat sources, require a long start up time.  HEHC engines can be designed small, start producing useful work as quickly as a regular gasoline or Diesel engine, and have the advantage of efficiency and fuel versatility.  HEHC engines can run on gasoline, jet fuel (JP8), diesel, natural gas, to name a few.  This opens up the possibility of using a much broader variety of fuel blends to maximize efficiency and operation in a variety of conditions such as cold weather.

For example, if one wanted to stop diesel from gelling in freezing temperatures, a blend of gasoline and diesel could be used--a feat normally impossible without causing severe damage to an internal combustion engine.  The design eliminates knocking and so requires no spark plugs, nor a forced fluid cooling system--it is air cooled, requiring no radiator.  Part of the reason for this, is that the engine turns more heat energy into mechanical work, rather than needing to sink it into the radiator.

LiquidPiston's design has really helped restore my hope in heat engines.  And it's not total baloney like those stories of cars that ran on water, that are about as silly as perpetual motion machines.  This one actually has a published paper hosted by MIT describing the technology.  Before this, I truly though heat engines were going to go the way of the dinosaur.  It now seems there may yet be progress in taking a turn-of-the-century technology, and bringing it up to speed with modern advancements.  It will be exciting to see the anticipated video of the X2 prototype engine operating.  The ideal 75% efficiency means that there might still be room to improve the real world design's efficiency into the 60's.  I would have never expected a heat engine that small to exceed 40%, let alone the near 60 of the current design and the surprising fuel versatility.  Hopefully the company won't be purchased by an automaker and disappear, like what happened with a certain mom-and-pop battery manufacturer in Detroit.