|LPI’s HEHC cycle. Click to enlarge.|
LiquidPiston, Inc., the developer of an efficient naturally aspirated rotary engine that can fire on both gasoline and diesel (earlier post) has closed a $5-million Series B investment round. Northwater Capital and Adams Capital Management were joined by unnamed angel investors.
The LiquidPiston engine is based on a thermodynamic cycle called the “High-Efficiency Hybrid Cycle” (HEHC), which borrows elements from Otto, Diesel, Atkinson, and Rankine cycles. The HEHC thermodynamic cycle has a 74% thermal efficiency limit, significantly higher than Otto or Diesel cycles on which most engines operate, according to the company. The company has progressed through a series of concept prototypes, and is on the M2.5 engine.
|LPI engine. Click to enlarge.|
LiquidPiston’s sub-50 horsepower designs are tailored for markets that are easier and quicker to enter, including portable gensets, Auxiliary Power Units (APUs), and Electric Vehicle (EV) range extenders.
Many unproven technologies with immature ecosystems are attracting attention right now, but it will be decades before combustion engines and ecosystems that support them surrender their prominent role in our economy. So much can be gained by making ICEs smaller, lighter, more fuel efficient, and hence more ecologically friendly. It would be a pity to falsely presume that combustion engines can’t be made even better.—cofounder and inventor Nikolay Shkolnik
Elements of the High Efficiency Hybrid Cycle (HEHC) cycle include:
Air compression to a high ratio (>18), followed by fuel injection and compression ignition (Diesel).
Constant volume combustion (Otto. Fuel is injected into the combustion chamber and auto-ignites, also similar to the Diesel engine. However, instead of immediately expanding, the burning fuel and gas mixture is constrained so that combustion occurs under constant volume. The combustion is held under isochoric conditions, and the mixture burns for an extended duration of time before the expansion stroke begins.
Over-expansion (Atkinson). Combustion products are expanded to near-atmospheric pressure, which requires a larger expansion volume than compression volume.
Internal cooling via water injection (Rankine). A small amount of water may (optionally) be injected during combustion or expansion. Water may facilitate the cooling, lubricating, and sealing of combustion chamber and pistons. Water used for cooling from within the engine turns to steam, which in turn increases the cylinder pressure, allowing for some of the cooling losses to be recuperated.
The combination of high compression ratio, true constant volume combustion, expansion into a larger volume than intake, and (optionally) water turning to high pressure steam cumulatively add to the efficiency of the engine. An air-standard analysis predicts thermodynamic efficiency of 75%.
LPI’s engine is architected to have the following properties:
- High fuel efficiency of 57% at peak, and 50% at part-load;
- High power density of 1 Hp / lb;
- Simple design with low part count;
- Multi-fuel capable: diesel, JP8, gasoline, natural gas, biofuels; and
- Low noise: the engine has low pressure exhaust and no poppet valves.
Stephen Nabours et al. (2010) High Efficiency Hybrid Cycle Engine (SAE 2010-01-1110)