Engines in relation to UHC emission

In terms of unburned hydrocarbon emission formation, several engine categories are available.

Unburned hydrocarbon emissions of CI engines, mostly diesel engines, are attributed to the excess air ratio. In the too lean region of fuel spray, the flame stops propagating, and in the rich area of the fuel jet, the air is not sufficient to burn all the available fuel. Both of these conditions, too lean and too rich mixture, are the source of unburned fuel, and consequently, a probableUHCsource. Moreover, this compound in a diesel engine is highly dependent on the fuel quality and the ignition delay [171–174]. When the ignition delay increases, a higher portion of the fuel will be in the over-lean area and results in theUHCformation [139]. A poor mixture formation due to the large droplets, low injection velocity, and cold crevices regions may also influence the total volume of UHCin diesel engines.

The fuel spray-wall interaction is also reported as an influential potential source of UHCfor diesel engines [175].

Dual-fuel engines exhibit higher UHC emissions than diesel, particularly at part-load conditions. During the full loads, the increase in the mixture strength and the improvement in the fuel utilization cause a reduction in totalUHCemission, but the quantity is still higher than that of conventional diesel mode [176]. There are mainly four mechanisms in dual-fuel engines that result inUHCformation:

crevice volumes, flame quenching, absorption and desorption of fuel in lubrication oil film, and the amounts of fuel remained in nozzle sac volume [177,178].

In premixedSIengines, since the fraction of the unburned fuel may even reach 5%

of total fuel, theUHCformation plays a more significant role in the combustion efficiency and totalGHGemission of the engines [109]. In premixedSIengines, unburned hydrocarbon originates from various sources. The importance of the sources changes by the fuel phase, whether it is liquid or gaseous. Liquid fuel vaporization carburetor [179], oil-films, and wall wetting in the cold start [113, 180,181] are reported as the main sources for liquid fuelSIengines. Injection of the liquid fuel in different places of the intake port showed a difference inUHC quantity. The liquid fuel entering close to the intake valve causes three times more UHCthan the farthest injection probe from the exhaust valve. Heating by the hot residual gas back-flow that occurs at the intake valve opening reduces the estimated UHCemission of the liquid source [182]. It was also confirmed that liquid fuel flow produces between 3 to 7 times higher amounts ofUHCthan vapor fuel.

This is worse when the injection is direct. In theDIfuel injection system, liquid fuels will be injected directly into the chamber, and liquid drops will evaporate during moving toward walls. Depending on the chamber pressure and temperature, the vaporization rate varies, specifically during cold start when the wall is colder than normal operating conditions. The sources of unburned hydrocarbon emissions inDIengines also depend on the engine load and speed, where injection timing changes to provide homogeneous optimized combustion [183]. With an early

be captured in the crevice volume, while with a late injection, the spray collides the piston surfaces and causes wall wetting before mixing in the air.

Hydrocarbon emissions arising fromHCCIengines are expected to be the crevice volume, the fuel escaped from the primary combustion process, flame quenching, and the absorption and desorption of fuel vapor into oil layers of cylinder wall [184]. Moreover, it was shown that most of the unburned hydrocarbons from a port injectedHCCIengine come from crevices [185].

Sources of UHC or methane slip in spark-ignition engines fuelled with natural gas are also reported as overlap, misfire, flame quenching, and crevices [186].

Changing a gasoline engine of vehicle application to a compressed natural gas engine showed a 50% reduction in terms ofUHC[73]. The main reason for this change was the reduction ofUHCin the oil film adsorption-desorption phenomena.

Less wall fuel flow in the intake system of lean burn engines has also contributed to the gaseous engine having less UHCcompared with a gasoline engine [72];

however, with the same power output compared with the gasoline engine, a gas engine was reported to have a 162% increase inUHC[187].

In general, seven classifications can be introduced for sources ofUHCof internal combustion engines. Any specific type of fuel or combustion may consist of one or more sources inUHCemission production.

1. Crevice volume [188]

2. Wall layer quenching [185]

3. Pockets of partially reacted mixture [186]

4. Misfiring [189]

5. Oil films [190]

6. Deposits [191]

7. Overlap [192]

Figure4.2 presents a flow chart of the distribution of the fraction of each of the sources on unburned hydrocarbon emission for a typical premixed spark-ignition gasoline engine. This flowchart can extensively stand for theSInatural gas engine except that the oil film and deposits have less influence than the other sources.

Figure 4.2: Flow chart mechanism of unburned hydrocarbon formation for a typical gasoline engine in steady-state [10].

As widely documented in the literature, theUHCsources are dominated by the fuel phase, combustion type, and injection properties. For a natural gas engine, where the fuel is in a gaseous mode and premixed with an injection valve before the intake valve, with some essential simplification, the UHCsources can be summarized into overlap, flame quenching, and crevices. Starting the initial flame by utilizing a spark plug and pre-chamber provides a high momentum gas jet entering the main chamber with good penetration. Thus, a misfire in a lean mixture with a pre-chamber rarely occurs during the normal operating condition.