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A free piston internal combustion (IC) engine operating on high compression ratio (CR) homogeneous charge compression ignition (HCCI) combustion is being developed by Sandia National Laboratories to significantly improve the thermal efficiency and exhaust emissions relative to conventional crankshaft-driven SI and Diesel engines. A two-stroke scavenging process recharges the engine and is key to realizing the efficiency and emissions potential of the device. To ensure that the engine's performance goals can be achieved the scavenging system was configured using computational fluid dynamics (CFD), zero- and one-dimensional modeling, and single step parametric variations. A wide range of design options was investigated including the use of loop, hybrid-loop and uniflow scavenging methods, different charge delivery options, and various operating schemes. Parameters such as the intake/exhaust port arrangement, valve lift/timing, charging pressure and piston frequency were varied.

A free piston, internal combustion (IC) engine, operating at high compression ratio (∼30:1) and low equivalence ratio (ϕ∼0.35), and utilizing homogeneous charge compression ignition combustion, has been proposed by Sandia National Laboratories as a means of significantly improving the IC engine's cycle thermal efficiency and exhaust emissions. A zero-dimensional, thermodynamic model with detailed chemical kinetics, and empirical scavenging, heat transfer, and friction component models has been used to analyze the steady-state operating characteristics of this engine. The cycle simulations using hydrogen as the fuel, have indicated the critical factors affecting the engine's performance, and suggest the limits of improvement possible relative to conventional IC engine technologies.

Sandia National Laboratories has been investigating a new, integrated approach to generating electricity with ultra low emissions and very high efficiency for low power (30 kW) applications such as hybrid vehicles and portable generators. Our approach utilizes a free piston in a double-ended cylinder. Combustion occurs alternately at each cylinder end, with intake/exhaust processes accomplished through a two stroke cycle. A linear alternator is mounted in the center section of the cylinder, serving to both generate useful electrical power and to control the compression ratio by varying the rate of electrical generation. Thus, a mechanically simple geometry results in an electronically controlled variable compression ratio configuration. The capability of the homogeneous charge compression ignition combustion process employed in this engine with regards to reduced emissions and improved thermal efficiency has been investigated using a rapid compression expansion machine.

In the continuing development of a hydrogen fueled IC engine optimized for application to a generator set or hybrid vehicle, experiments were performed at Sandia National Laboratories on two engine configurations. The intent is to maximize thermal efficiency while complying with strict emissions standards. The initial investigation was conducted utilizing a spark ignited 0.491 liter single cylinder Onan engine and has progressed to a spark ignited 0.850 liter modified for single cylinder operation Perkins engine. Both combustion chamber geometries were “pancake” and achieved a compression ratio of 14:1. The engines were operated under premixed conditions. The results demonstrate that both engines can comply with the California Air Resources Board's proposed Equivalent Zero Emission Vehicle standards for NOx during operation at an equivalence ratio of 0.4.

Engine dynamometer testing of homogeneous charge, spark ignition lean burn engines fueled by natural gas, hydrogen/natural gas blends and neat hydrogen was conducted to determine if NOx emissions from blended fuel operation can be reduced below those generated from natural gas operation, approaching those due to a 100% hydrogen fueled engine. The preliminary tests were conducted at the University of Central Florida/Florida Solar Energy Center on an eight cylinder automotive engine. The results indicate that the hydrogen/natural gas fuel has the potential of meeting highly restrictive NOx levels. Sandia National Laboratories conducted follow-on, comparative tests using a single cylinder research engine. The Sandia results indicate that the proposed CARB EZEV standard for NOx can be met without exhaust gas aftertreatment using a 30% hydrogen (by volume) / 70% natural gas blend fuel in a constant speed/power, hybrid vehicle application which achieves 60 MPG gasoline equivalent efficiency.

Lawrence Livermore, Sandia Livermore and Los Alamos National Laboratories have a joint project to develop an optimized hydrogen fueled engine for series hybrid automobiles. The major divisions of responsibility are: system analysis, engine design, and kinetics modeling by LLNL; performance and emission testing, and friction reduction by SNL; computational fluid mechanics and combustion modeling by LANL. This project is a component of the Department of Energy, Office of Utility Technology, National Hydrogen Program. We report here on the progress on system analysis and preliminary engine testing. We have done system studies of series hybrid automobiles that approach the PNGV design goal of 34 km/liter (80 mpg), for 384 km (240 mi) and 608 km (380 mi) ranges. Our results indicate that such a vehicle appears feasible using an optimized hydrogen engine. The impact of various on-board storage options on fuel economy are evaluated.