Each laser-based “spark plug” consisted of an 807µm laser-diode source that then “pumped” a laser for increased output power. The pumping action used 250µs pump-pulse duration with a repetition rate of about 100 Hz. The final output was coupled to the engine cylinder via a 600µm optical fiber. The overall arrangement delivered 4mJ pulses at a 1.06 µm wavelength and 0.8ns width to the cylinder. For the laser energy to reach within the cylinder, each was fitted with a sapphire window attached with special epoxy adhesive. The adhesive could withstand operating temperatures from −70 to +170°C. The laser beam was focused inside each cylinder at the same location where a conventional ESP creates its spark.
The arrangement was tested under two fuel conditions on a four-stroke, four-cylinder, multipoint fuel-injection gasoline passenger car engine having a standard 30 mJ ESP versus the LSP arrangement – see figure 2.
Using a stoichiometric air-fuel ratio (λ~1) indicating no unburned fuel or air, the LSP approach yielded nearly an 8% increase in engine brake power, a 7.5% reduction in brake-specific fuel consumption (BSFC), and a 20% reduction in carbon-monoxide (CO) emissions – see figure 3.