Sandians’ pursuit of ideal engine cycle for electricity generation wins honors
As he investigated creating electricity with a crankshaft engine/generator that burns hydrogen fuel, Peter Van Blarigan of Engineering for Emerging Technologies Dept. 8118 wondered if there was a better way to generate electricity in an internal combustion device.
His investigation of this new approach is being honored out of hundreds of submitted papers as the single most important contribution to the Society of Automotive Engineers in the fields of combustion, engines, and fuels for the year 1998. Peter will receive the Harry Lee Van Horning Award Oct. 26 at the annual SAE Fall International Fuels and Lubricants meeting.
"It was totally unexpected because the work is unconventional," Peter says. "In addition to investigating conventional technology, it’s good to look at a few new things like this."
Unique environment facilitates work
One of the awards committee members told him the paper showed particular care, precision, and quality of experimental work unique to the national laboratory environment.
A mechanical engineer, Peter received three years of Laboratory-Directed Research and Development support, as well as project funding from DOE’s Office of Solar Thermal, Biomass Power and Hydrogen Technologies. Two students, Nicholas Paradiso and Scott Goldsborough (both 8118), are assisting the project as they complete their doctoral work in his lab.
The team modified old pressure vessels and used existing electrical measurement equipment to build a single-cycle experiment in order to investigate the unique combustion system.
"It’s mechanically simple," Peter explains, "but 20 years ago, it probably couldn’t have been built. Enabling aspects include advanced electrical controls, new magnet technology and today’s emphasis on emissions."
In the device, a piston in a cylinder bounces between two combustion chambers. Permanent magnets are attached to the piston, generating electricity by passing through the coils of an alternator centered on the cylinder.
Since the piston can move freely, the compression ratio can be varied by how much energy is taken from the alternator — the more energy removed, the less the piston travels toward either end of the cylinder. The result is electronic control of compression ratio.
To power the device with maximum thermal efficiency, fuel and air are completely mixed in advance. The mixture is highly compressed at each end of the cylinder so that it becomes hot enough to self-ignite rapidly without flame propagation. During combustion, the cylinder volume remains essentially constant — a key to its efficient conversion of thermal energy to mechanical work.
An ideal state of performance
The device seeks to achieve a theoretically ideal state of performance, called the Otto cycle. Peter has given it the technical name "free piston linear alternator with homogenous charge compression ignition."
Peter’s group tested such fuels as propane, natural gas, hydrogen, methanol, and light, easily vaporized hydrocarbons. He found that lean fuel mixtures at a high compression ratio of about 30:1 improved thermal efficiency by nearly 40 percent compared to the standard compression ratio (10:1) of spark-ignition engines. The results for propane and natural gas were about 56 percent indicated efficiency. The primary cause of this high conversion efficiency, he said, is nearly constant volume combustion at a high compression ratio.
Keeping the fuel mixture lean and homogeneous helped limit formation of oxides of nitrogen (NOx) and particulates. Treating exhaust gases with an oxidation catalyst, meanwhile, could control emissions of hydrocarbons and carbon monoxide. Low emissions are a primary driver for investigating this approach, he said. The balance of high thermal efficiency and relatively low emissions could make the concept an attractive alternative to diesel electrical power generation.
Hybrid vehicle applications
Applications include using such a device as an auxiliary part of the power train in a hybrid vehicle. Hybrid vehicle schemes entail generating and storing electricity on-board, using a small engine to save fuel and limit emissions. Another application might be use in a distributed power generation network. Peter said he thinks the device offers the advantage of improved durability because there is no crankshaft to add mechanical stress from piston side loading.
Advances in magnet technology also permitted creation of the concept, since dramatically stronger "rare earth" magnets of neodymium-iron-boron permit use of a reasonably small alternator. The General Motors spin-off company Magnaquench is working with Peter’s team to develop an alternator using the company’s permanent magnet expertise.
The team is also working with chemical kinetics modelers at Lawrence Livermore National Laboratory, where one experimental result was not predicted — Peter found that for some hydrocarbons up to 15 percent of the fuel was left unconsumed in the single-stroke experiments.
He next plans to demonstrate electrical generating efficiency and emissions in a research prototype. Three other Sandians have received Horning awards over the years John Dec (8362), Peter Witze (8362), and Mike Dyer (8700).