LiquidPiston, a developer of highly efficient internal combustion engines, has raised $6.5 million in a round of venture capital, according to a new filing by the company with the Securities and Exchange Commission. Six separate investors participated in the offering. Current investors in LiquidPiston include Northwater Capital and Adams Capital Management, among others.

LiquidPiston develops internal combustion engines based on an innovative thermodynamic cycle, increasing average efficiency over conventional diesel engines from less than 20% to more than 50% under typical operating modes. LiquidPiston engines are compact, quiet and powerful, with a lower total carbon footprint for environmental sustainability. The engines are also multi-fuel capable.