Automotive Evaporative Emission Control

ABSTRACT

           The diesel engine has provided the power units for transportation system, i.e. buses, and goods transportation system, i.e. trucks, Indeed the petrol engine powered automobile and diesel engine powered buses and trucks are the symbols of modern technological society.

           

However, in recent time the internal combustion engine powered vehicles have come under heavy attack due to most serious problem of air pollution. From these powered vehicles different pollutants like carbon monoxide, unburned hydrocarbons, oxides of nitrogen and other particular emissions causes air pollution.

           

The evaporate loss both tank and carburetor are possible sources of atmospheric pollution from a petrol engine powered vehicle. The evaporative emission from fuel tank causes fuel tank losses. These losses are due to less the tank fill; due to temperature increases etc. similarly the carburetor losses are due to evaporative emission from carburetor.

All above explain different types of losses causes the air pollution. The air pollutants by automobile are 50% of total air pollutants.

It is very necessary to over come this serious problem by using the evaporation loss control device (ELCD). The ELCD completely controls all types of evaporative losses.                

              

INTRODUCTION

           The rise in civilization is closely related to improvements is transportation. In the development of transport the internal combustion engines, both petrol and diesel engines, occupy a very important position. The petrol engine has provided reliable small power units for personalized transport and in this way revolutionized the living habits of people to a great extent. The diesel engine has provided the power units for transportation system, i.e. buses, and goods transportation system, i.e. trucks, Indeed the petrol engine powered automobile and diesel engine powered buses and trucks are the symbols of modern technological society.

           However, in recent times the internal combustion engine powered vehicles have come under heavy attack due to various problems and most serious problem is air pollution. The main pollution contributes by automobiles are carbon monoxide (CO), unburned hydrocarbons (UBHC), oxides of nitrogen and other particular emissions. However, in advance countries like U.S.A. the air pollutants by automobiles are about 50 per cent of the total air pollutants. The pollutants amount becomes millions of tonnes. In U.S.A. the number of petrol vehicles were approximately 10 crore in 1973 and may rise to 20 crores by the turn of the century. The estimated total pollutants emitted by them in 1969 were 110 million tonnes of CO, 20 million tonnes of HC and 10 million tonnes of No_x. Thus it is imperative that serious attempts should be made to conserve earth’s environment from degradation.

POLLUTANTS FROM GASOLINE ENGINES

           

There are four possible sources of atmospheric pollution form a petrol engine powered vehicle: the fuel tank, the carburetor, the crankcase and the exhaust pipe.

            The contribution of pollutants, by source, as follows.

a)      Evaporative loss (both tank and carburetor)               15 to 25% of HC

b)      Crankcase blowby                                                       20 to 35% of HC

c)      Tail pipe exhaust                                                         50 to 60% of HC and   

                                                                                                Almost all CO and No_x

            The evaporative losses are the direct losses of raw gasoline from the engine fuel system.

EVAPORATIVE LOSSES

            Evaporative emissions account for 15 to 25 per cent of total hydrocarbon emission form a gasoline engine. The two main sources of evaporative emissions are the fuel tank and the carburettor.

·         FUEL TANK LOSSES

Fuel tank losses occur by displacement of vapour during fillings of petrol tank, or by vaporization of fuel in the tank, forcing the vapour through a breather vent to the atmosphere. Fuel tank losses occur because the tank temperature is increased during the vehicle operation which causes an increase in the vapour pressure and thermal expansion of tank vapour.

·         EMISSIONS FROM EVAPORATIVE LOSSES FROM THE CARBURETTOR AND THE FUEL TANK

These results from the dissipation of heat when the engine is turned off and from variations in temperature between day and night.  The following equations have been established. Engine stopped:  % weight of benzene in vapour = 0.45 of the weight of benzene in petrol Respiration : % weight of benzene in vapour = 0.89 of the weight of benzene in petrol.

EMISSIONS FROM  EXHAUST 

            BZ_exh = 0.50 +0.44 BZ_gas + 0.04 Ar
            Where

BZ_exh                 Is the percent weight of benzene in the exhaust

            BZ_gas                 Is the percent weight of benzene in petrol (gasoline)
            Ar                    Is the percent weight of other aromatics in petrol (gasoline)

For Benzene exhaust from petrol vehicles, separate equations were used for three-way catalysts, three way plus oxidation catalysts, and other catalyst types.  For vehicles with a three- way catalyst, running on baseline gasoline, the following equation was used

            3-way Bz%THC            = 1.077+0.7732* (Volume % benzene)
                                                  + 0.0987* (Volume % aromatics – volume % benzene)

This equation was obtained by the EPA Regulatory Development and Support Division (RDSD) from work done by Chevron Oil Company. For vehicles with a three-way plus oxidation catalyst, running on baseline petrol the equation used was

            3-way + oxBz%THC            = 0.6796* (Volume % benzene)
                                                   +0.0681* (Volume % aromatics – volume % benzene)

This equation was obtained from the draft Regulatory Impact Analysis for RVP regulations (EPA, 1098 a).

·         MECHANISM OF TANK LOSS

           

When a partially filled fuel tank is open to atmosphere the partial pressure of the vapour phase hydrocarbons and vapour pressure of the liquid are equal and they are in equilibrium. If the temperature of the liquid is increased, say by engine operation, the vapour pressure of the liquid will increase and it will vaporize, the total pressure of the tank increases and since the tank is open to atmosphere the vapour will flow out of the tank. This, outflow to the vapour will increase if in addition to liquid temperature rise, the vapour temperature is also increased.

            Less the tank fill, greater is the evaporation loss. The effects of tank fill and temperature are shown in table. 

 

Tank fill	Ambient temp 0C	Temp. rise during test, 0C	Loss during operation
1 / 4	19	7	5.7
1 / 2	16	4	1.2
3 / 4	18	2	0.1
Full	22	3	0.0

            When a car is parked in a hot location the evaporation of the gasoline in the accelerates, so the evaporation loss is greater. The vapour which vent from a partially filled tank during vehicle operation called soak, is a mixture of air and hydrocarbon.

·         CARBURETTOR LOSSES

Carburettor losses result from external venting of the float bowl relieving the internal pressure as the carburettor heats and “ hot soak ” losses which occur after the engine has been stopped, as a result of evaporation of petrol stored in the bowl, loss being through vent pipe or through the air cleaner. Most of the loss from the carburettor occurs due to direct boiling of the fuel in the carburettor bowl during hot soak. If the pressure in the fuel line becomes greater than the pressure holding the needle valve closed, after supply will occur. One of the possible reasons may be fuel evaporation pressure in the carburettor bowl which presses down the bowl and increase pressure in the fuel line. If the after supply is more than the bowl volume the losses from the carburettor will change drastically.

HOT SOAK EMISSIONS

            Hot soak emissions are the gasoline vapours generated immediately following shutdown of an engine due to vaporization of the fuel remaining in the carburetor float bowl as it is warmed by the residual heat of the engine.

  

HOT SOAK SOURCE OF THE VALUE USED IN NONROAD

           

            The beta release of NONROAD does not include any estimates of hot soak evaporative emissions. There are two reasons for this. First, the limited data that are available indicate that hot soak emissions are minimal relative to other types of hydrocarbon emissions on the order of 1 % of total HC emissions from gasoline fueled nonroad engines. As described in the following section on other methods and data, the available hot soak data for nonroad engines is limited to rather inconclusive data from just 8 engines in a narrow power range and values available from highway engines are not considered representative of nonroad engines due to the different size, design, packaging, and fuel metering systems involved.

           

            Thus, until more data become available, the Nonroad Engine Emissions Modeling Team (NEEMT) plans to follow the same approach used in NEVES as well as in the EPA Phase I small engine regulatory model and in the CARB OFFROAD model namely to not include hot soak emission factors in the model. However, the model code is written to allow for the addition of a hot soak emission data file if such estimates become available.  

      

EVAPORATION LOSS CONTROL DEVICE (ELCD)

             This device aims at controlling all evaporative emissions by capturing the vapours and recirculating them at the appropriate time. The device as shown in fig.

It consists of an adsorbent chamber, the pressure balance valve and the purge control valve. The adsorbent chamber, which consists of a charcoal bed or foamed polyurethane, holds the hydrocarbon vapour before they can escape to atmosphere. The carburettor bowl and the fuel tank, main sources of HC emissions, are directly connected to the adsorbent chamber when engine is turned off i.e. hot soak condition when a warmed up car is stopped and its engine turned off. This results in some boiling in the carburettor bowl and significant amount of HC loss occurs. Thus the hot soak loss and chamber and are adsorbed there. The adsorbent bed when saturated is relived of the vapours by a stripping action allowing the air form the air cleaner to draw them to the intake manifold through the purge valve. The internal seat of the pressure valve at that time is so located that there is a direct pressure communication between the internal vent and the top of the carburettor bowl, maintaining designed carburettor metering forces.

            The ELCD (evaporation loss control device) completely controls all types of evaporative losses. However, the tolerance of the carburettor for supplying F/A ratio reduces to about 3 per cent only. This requires very accurate metering control.

REFERENCES 

·         BOOKS

·         M. L. Mathur, R. P. Sharma, Internal combustion engines. Dhanpat rai publications, 2002.

·         NONROAD engine & vehicle emission study with appendixes, US environment protection agency.

·         Website

www.macaple.com