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Multiple Injection Strategy in a Direct-Injection Natural Gas Engine with Entrained Diesel
Abstract:
A new fuel injector prototype for heavy-duty engines has been developed to use direct-injection natural gas with small amounts of entrained diesel as an ignition promoter. This “co-injection” is quite different from other dual-fuel engine systems, where diesel and gas are introduced separately.
Reliable compression-ignition can be attained, but two injections per engine cycle are needed to minimize engine knock. In the present paper the interactions between diesel injection mass, combustion timing, engine load, and engine speed are investigated experimentally in a heavy-duty single-cylinder engine.
For the tests with this injector, ignition delay ranged from 1.2–4.0 ms (of which injector delay accounts for ~0.9 ms). Shorter ignition delays occurred at higher diesel injection masses and advanced combustion timing. At ignition delays shorter than 2.0 ms, knock intensity decreased with increasing ignition delay. Ignition delays longer than 2.0 ms indicated that the pilot gas injection did not ignite before large amounts of fuel were injected with the second injection; intense knock could occur in these cases.
At low load, carbon monoxide (CO) and total hydrocarbons (tHC) emissions were anti-correlated with the diesel injection mass. Oxides of nitrogen (NOx) emissions depended strongly on timing, but not on diesel fuelling. At higher load, emissions of CO and tHC were very low and showed no clear correlation with diesel fuelling. At advanced timing where the pilot gas injection was not observed to ignite (such as for the low-diesel runs) NOx emissions were higher.