Study on the effect of fuel additive on PFI gasoline engine performance and emissions

. With increasingly stringent emission and fuel consumption regulations, fuel additives have been proved to have a huge role in energy conservation and emission reduction, a new type of fuel additive based on nitroparaffin has received worldwide attention. An experimental study was conducted with the aim of developing an understanding of the fuel additive’s effect on a 1.5L naturally aspirated PFI gasoline engine’s power performance, fuel economy, emissions and cylinder cleanliness. The results show that when fueled with fuel additive, engine power increased at full load condition. The commonly used working condition’s average fuel consumption decreased by 4.5% at fuel consumption contour map. For pollutant emissions, unburned hydrocarbons (HC), smoke (filter smoke number, FSN) decreased significantly, slightly higher nitrogen oxides (NOx) compared with no fuel additive. Furthermore, the use of fuel additive has a positive effect on elimination of carbon deposit in the cylinder.


Introduction
Automobile emission is an important source of air pollution, more and more attention has been paid to it.The engine emissions are derived from the fuel's combustion, so using the high-quality fuel and optimizing the combustion process can reduce harmful emissions and improve fuel consumption [1] .The quality level of fuel is mainly determined by the petrochemical production process, the combustion process optimization is due to the advanced engine technology application.While the fuel quality level and engine technology under the established condition, the use of relevant fuel additives can also achieve a positive effect on its combustion performance, thereby improving the engine emission performance, power economy and cylinder cleanliness [2][3] .The combustion process of an internal combustion engine determines the emissions of pollutants, and fuel additives can optimize the combustion process, reduce exhaust emissions and ensure compliance with increasingly stringent emissions regulations.
MAZ is a China's patented chemical formulation consisting of a liquid concentrate blended with high-energy organic hydrocarbons for dosing in fuels to achieve higher performance objectives.MAZ releases extra chemical energy from the fuel converting it to higher heat value to increase mechanical power at the wheels.In addition, the fast and higher temperature burn of MAZ delivers an effective engine detergency from a nitroenriched combustion process rather than through the usual "chemical detergency process" arising from amines-based detergency control additives.MAZ is a nitro-alkane based concentrate noted for its enhanced combustion power to produce more effective engine detergency.Most importantly, during the combustion process, MAZ also generates a unique molecular protective film on the metal surface inside the combustion, which reduces engine friction and heat loss.The protective film promotes greater mechanical efficiency to enable the engine to operate more efficiently and for longer lifespan.
A type of fuel additive was designed to reduce tailpipe, its main components are a combination of nitroparaffins.It consists of 40-65% nitropropane, 15-25% nitroethane, 15-25% nitromethane, 5-15% toluene, 0.5-5% L-1699(a proprietary blend of Exxon-Mobile Chemical Company).It exhibits high heat value, excellent carbon deposit prevention, lubricity and high chemical reactivity that results in the development of free radicals in the course of the combustion process.This, in turn, initiates a chain reaction providing more complete combustion.This results in lower tail pipe emissions and fuel economy.Fuel additive as a new type of fuel additive has received worldwide attention, which has low water solubility, is biodegradable and contains no metallic substances making it environmentally friendly [4] .
Studys show that the existing high octane number oxygenates unable to meet the requirements of safety and environmental protection, such as MTBE, because MTBE is soluble in water and resists biodegradation resulting in groundwater pollution [5][6][7] .Furthermore, MTBE mainly through the respiratory tract absorption, also by the skin and digestive tract, high concentrations of MTBE can be carcinogenic.Since May in 2006, 25 states have banned MTBE in the United States.MMT affects the proper functioning of catalytic converters and on board diagnostic computer systems.Compared with other fuel additives, Fuel Additive is more biodegradable and less water soluble, making it more effective and safer alterative [4] .
University of California's Boums [4] conducted a study of fuel additive's effect on engine's fuel economy and emissions under the chassis dynamometer system.Results show that compared with the normal benzene with no fuel additive, the use of fuel additive can improve the engine's power, fuel economy and emission performance.The results show that fuel additives have positive effects on emission reduction and energy saving.
Lanzhou Refinery Chemical Company [8] conducted a road trial using 3 cars for the study of fuel additive.The test used RON 90 and RON 93 high olefin content gasoline fuel (45% olefin content) treated with fuel additive.The raod trial covered 23000km, including highland, desert and rough road.Results show that the fuel consumption per 100km decreased 2.87%-11.11%,HC and CO emissions reduced, while NOx emission increased slightly.During the road trial, the driver sensed a noticeable increase in power after fuel additive was added to the fuel.
South China University of Technology [4] conducted a road trail test on motorcycle using RON 90 high olefin gasoline added with 500ppm fuel additive.Before and after the test, the engine cylinder head was opened to inspect and take pictures of the condition of the combustion chamber wall.Contrastive analysis shows that fuel additive has surface detergency effect.
Previous studies mainly focus on vehicle test, which is more complex and uncontrollable because many uncertain variables.Based on the engine test bench experimental study, this paper expounds the influence of single factor variable (with or without fuel additive) on engine performance, including four aspects:power performance, fuel economy, emissions performance and cylinder cleanliness.In this paper, the evaluation index system is perfected and the function of fuel additives is evaluated comprehensively.The evaluation indexes such as performance, emission, fuel economy and in-cylinder cleanliness were constructed to carry out comprehensive evaluation tests of fuel additives.

Test equipment and condition
Test engine main technology parameters are shown in Table 1.The test was conducted on engine bench, the test bench system diagram as shown in Figure 1.Fuel meter can measure engine's fuel consumption precisely, the dynamometer was used for measuring engine's torque and power.Emission devices included AVL 415 smokemeter and HORIBA OBS gas analyzer, which can measure and obtain engine exhaust's FSN and pollutant gas concentration respectively.Before and after the test, the engine's spark plug was removed to take pictures of combustion chamber and valves by using endoscopic to acquire carbon deposit degree.The base test fuel is China VI stage standard gasoline, the base fuel and fuel additive according to the mass ratio of 1000:1 for blending to get the fuel with fuel additive.

Test procedure
Comparative study of engine's power and emission performance on speed characteristics under full throttle condition, engine's fuel economy on mapping characteristics respectively.
Based on the above test conditions, test process as follow: Firstly, engine using base fuel conducted above test items under respective test conditions to acquire the evaluation indexes values.Then, engine using fuel with fuel additive run 25 hours continuously under full load at 2800r/min to ensure fuel circuit was flushed completely.At last, the same engine test items was conducted by using fuel with fuel additive under the same respective test conditions to get engine's power and fuel economy performance, emission characters.Before and after the fuel additive test, taking photos to observe the engine's combustion chamber and valves by using endoscopic to acquire carbon deposit degree.

Engine power performance analyse
Engine speed characteristics under full throttle condition can reflect its power performance, so the full load power characters was compared when engine using fuel with and without fuel additive, the results as shown in Figure 2. The results indicate that engine power increases under most of full load conditions when using fuel with Fuel Additive additive, the largest increase is about 1.5%.While the Fuel Additive additive makes engine power a slight decrease at high speed area above 5000r/min.This phenomenon is due to the Fuel Additive additive's effect on engine combustion progress.Fuel Additive additive's main components are a combination of nitroparaffins, which exhibits high heat value and chemical reactivity.It's easy to generate free radical in combustion progress, which in turn, initiates a chain reaction providing more complete combustion.So the engine using fuel with Fuel Additive additive can make combustion progress close to the constant volume, which increases the combustion efficiency.

Engine fuel economy performance
Fuel consumption rate is an important index for evaluating engine fuel economy.So the engine' fuel consumption rate at speed characteristics under full load was compared when engine using fuel with and without Fuel Additive additive, the results as shown in Figure 3. Results show that the minimum fuel consumption rate at full load when engine using fuel with and without Fuel Additive additive is 222.9 g/kWh and 236.1 g/kWh respectively.Fuel Additive additive can decrease the fuel consumption rate except the high speed above 5000 r/min condition, the maximum decrease can reach 5.6%.While the fuel consumption rate increases slightly above 5000r/min speed condition.
In order to evaluate engine fuel economy at common condition, comparative analyse of fuel consumption rate on mapping characteristics when engine using fuel with and without Fuel Additive additive respectively, the results as shown in Figure 4. From the fuel consumption rate on mapping characteristics, we can acquire the conlusion that Fuel Additive additive can decrease the fuel consumption rate and improve the fuel economy at most of the operating section.Furthermore, the use of Fuel Additive additive can expand the fuel economy zone in the direction of high speed and torque, which expands the fuel economy zone effectively.Choosing the common engine speed and torque range at the fuel consumption rate contour map, this is speed and torque range: 1200-4800r/min, 30-120Nm.The calculated average fuel consumption rate at this common zone are 271.3g/kWhand 284.2g/kWh when engine using fuel with and without Fuel Additive additive respectively.So compared with base fuel, engine using fuel with Fuel Additive additive can decrease the common operating zone's average fuel consumption rate by 4.5%.This is due to the Fuel Additive's high chemical reactivity promotes free radical generation and initiates a chain reaction, which provides the chemical reaction environment for more complete combustion.

Engine emission performance
In order to evaluate the influence of fuel additive' effect on gasoline engine emission performance, this experimental study conducted the engine bench test to acquire the exhaust emission smoke (filter smoke number, FSN) and emission gas when engine using fuel with and without Fuel Additive additive respectively.The emission gas includes total hydrocarbon (THC), carbon dioxide (CO) and nitrogen oxides (NOx).

Emission smoke (FSN)
FSN is the representation of particulate emission, its monotonicity and particulate emission quantity and quality are positively correlated.Results show in Figure 5. that Fuel Additive additive can effectively decrease engine exhaust emission smoke under full load condition, the maximum decrease by 66.5% at 5600r/min.However, compared with base fuel, engine emission smoke has a slight increase at low speed below 2800r/min when engine using fuel with Fuel Additive additive.From the whole full load curve, we can acquire the conclusion that Fuel Additive additive can decrease the emission FSN.

Engine gaseous emissions
In this experimental study, comparative analyse of the gasoline engine exhaust gaseous emissions (THC, CO and NOx) under full load conditions when engine using fuel with and without Fuel Additive additive respectively.Compared with base fuel, as shown in Figure6, engine THC emission decreases at whole speed range under full load condition when engine using fuel with Fuel Additive additive.The maximum decrease of THC emission is 28.3% at 4000r/min.After the combustion of the combustion agent is used, the combustion is more sufficient, the combustion temperature in the chamber is increased, and the emission of the unburned hydrocarbon is reduced.As shown in Figure 7, compared with base fuel, engine NOx emission increases at whole speed range under full laod condition when engine using fuel with Fuel Additive additive.Fuel Additive's high chemical reactivity promotes generation of free radical in combustion progress, which initiates a chain reaction to increase the flame propagation velocity and combustion temperature.Also the Fuel Additive has oxygen and nitrogen, those factors lead the NOx emission increase.So the Fuel Additive additive can increase the NOx concentration in gasoline engine exhaust.Considering in normal conditions, the gasoline engine NOx emission value well below the China Ⅴ stage emission standard limit, the three-way catalytic converter can also decrease the NOx concentration.So the Fuel Additive's negative influence on gasoline engine NOx emission is controlled.
As shown in Figure 8, the fuel additive added to gasoline can improve the mixture formation and combustion quality, then change and improve gasoline engine's combustion characteristics.At low speed and high speed range under full load condition, the CO emission decreases and increases respectively when engine using fuel with fuel additive.This phenomenon is due to the fuel additive increases the cylinder temperature at low speed condition, so the CO emission decreases.Engine load increases when speed increases at certain range, fuel additive has a strong ability to capture oxygen considering its high chemical reactivity, so oxygen concentration decreasing can be happened at high speed and torque.

Carbon deposit in engine cylinder
Carbon deposit worsen engine performance, so this study evaluated the fuel additive's effect on engine cylinder's carbon deposit.Before and after the test, the engine's spark plug was removed to take pictures of combustion chamber and valves by using endoscopic to acquire carbon deposit degree, which is used to evaluate fuel additive's effect of removing carbon deposit.
Carbon deposit degree in piston head, the contact point of piston and cylinder and valves were compared when engine using fuel with and without fuel additive respectively, the before & after photos as shown in Figure 9-11.From the compared photos of carbon deposit degree by endoscopic observation, carbon deposit degree in piston head, the contact point of piston and cylinder and valves were thinner or reduced significantly when engine using fuel with fuel additive compared with base fuel.Results show that fuel additive has a positive effect on eliminating the carbon deposit in cylinder.

Conclusion
(1) A comprehensive fuel additive evaluation method has been proposed on a 1.5L naturally aspirated PFI gasoline engine, which includes fuel economy, power, emission characteristics and engine carbon deposition.
(2) Engine power increases under most of full load conditions when using fuel with fuel additive, the largest increase is about 1.5%.Fuel additive can decrease the fuel consumption rate, improve the fuel economy at most of the operating section and expand the fuel economy zone in the direction of high speed and torque.Fuel additive can decrease the common operating zone's average fuel consumption rate by 4.5%.
(3) Fuel additive can effectively decrease emission smoke (FSN) under full load condition, the maximum decrease by 66.5% at 5600 r/min.Compared with base fuel, engine THC emission decreases at whole speed range under full load condition when engine using fuel with fuel additive.The maximum decrease of THC emission is 28.3% at 4000 r/min.While engine NOx emission increases at whole speed range under full laod condition.At low speed and high speed range under full load condition, the CO emission decreases and increases respectively when engine using fuel with Fuel Additive additive compared with base fuel.
(4) Test results show that fuel additive has a positive effect on eliminating the carbon deposit in cylinder.Fuel additive can reduce the carbon deposit in combustion chamber, intake and exhaust valves significantly.

Fig. 2 .
Fig. 2. Engine power performance with and without fuel additive.

Fig. 3 .
Fig. 3. Engine fuel consumption rate with and without fuel additive.

Fig. 8 .
Fig. 8. Engine CO emission with and without fuel additive.