Investigation of Early and Late Intake Valve Closure Strategies for Load Control in a Spark Ignition Ethanol Engine

The more strict CO2 emission legislation for internal combustion engines demands higher spark ignition (SI)engine efficiencies. The use of renewable fuels, such as bioethanol, may play a vital role to reduce not only CO2 emissions but also petroleum dependency. An option to increase SI four stroke engine efficiency is to use the so called over-expanded cycle concepts by variation of the valve events. The use of an early or late intake valve closure reduces pumping losses (the main cause of the low part load efficiency in SI engines) but decreases the effective compression ratio. The higher expansion to compression ratio leads to better use of the produced work and also increases engine efficiency. This paper investigates the effects of early and late intake valve closure strategies in the gas exchange process, combustion, emissions and engine efficiency at unthrottled stoichiometric operation. A four-valve four-stroke single cylinder camless engine running with port fuel injection of anhydrous ethanol was employed. Early and late intake valve closure (EIVC and LIVC) strategies with a fixed maximum valve lift were compared to a conventional throttled SI valve event strategy for loads from 2.0 to 9.0 bar IMEP at 1500 rpm. The consequences and benefits to implement the unthrottled operation with each strategy were discussed. To better understand the effect of the maximum valve lift at a specific load, the valve lift was varied from 1.5 to 5.0 mm and its effects were discussed for EIVC strategy. Comparatively, the EIVC strategy presented better overall performance than the LIVC. Both unthrottled strategies provided higher engine efficiency than the conventional throttled SI strategy. Introduction The more strict CO2 emissions legislation for passenger cars increased the need for more efficient spark ignition (SI) engines. Lower carbon footprint and reduced greenhouse gases (GHG) emissions are expected to reduce the climate change impacts. In this context, the use of environmentally friendly fuels with lower CO2 emissions, such as bioethanol, has been growing worldwide. Ethanol is generally produced from fermented sugar from diverse agricultural crops. It may reduce a country oil dependency and manage surplus of agricultural crop production [1], [2]. Depending on land usage management, ethanol life cycle GHG emission can be considerably lower than that from fossil fuels[3], [4]. For this reasons, the introduction of ethanol in many countries has increased in the last decades. Even then, the international oil price, internal crop availability and sugar prices dictate the ethanol production and consumption in the larger producer countries, such as the United States and Brazil. Ethanol has been used both as a dedicated fuel and as gasoline antiknock additive for SI engines. In many countries, flex fuel engines permit the use of any ethanol-gasoline blend. The use of ethanol and ethanol-gasoline mixtures in SI engines has been widely reported [5]–[9]. Some ethanol advantages over gasoline are the increased knock resistance and increased heat of vaporization which may lead to higher engine efficiency. Conversely, the higher heat of vaporization decreases the engine cold start capability and the lower ethanol energy content increases the volumetric fuel consumption compared to gasoline. In order to increase the naturally aspirated SI engine part load low efficiency, distinct strategies can be used. Lean burn and exhaust gas recirculation (EGR) may be employed to dethrottle the engine and reduce pumping losses. While lean burn highly increases the complexity of the exhaust after treatment system, EGR can be used in various ways and even enhance the after treatment system performance. In addition, the use of Miller and Atkinson cycles, based on early or late intake valve closure, can also be applied reduce pumping losses. As the intake valve closure point is moved away from bottom dead center (either earlier or later), less air is trapped in the cylinder leading to less energy released in a stoichiometric combustion. Therefore, variable valve closure strategy at wide open throttle can be used as load control method. As demonstrated in the literature, this may highly reduce the part load pumping losses while affecting the incylinder flow structures and turbulence levels[10]–[17]. Considering the two main large in-cylinder flow structures swirl and tumble, studies have shown that these large flow motion scales break up in small scales during the late stage of compression increasing the turbulence during combustion [18], [19]. The tumble motion is the large scale fluid motion generated during the intake stroke around an axis perpendicular to the cylinder center line. While the piston is moving towards TDC, during compression, the tumble motion initially increases due to angular momentum conservation. Later, during compression stroke, the large flow structure is distorted due to wall shear stress and decays in smaller turbulence structures[20]–[22]. Swirl is the rotational fluid motion around the cylinder axis. Conversely to tumble, the swirl motion is less affected by wall friction and hence its angular momentum can be well sustained until the end of the compression stroke [23]. So, in four-valve SI engines with symmetric configuration, the increase of the tumble in-cylinder motion is expected to generate higher turbulence levels prior to combusPage 2 of 20 7/20/2015 tion than the increase in swirl [24], [25]. Even then, if not enough tumble motion is generated e.g at mid-low engine loads, poorer turbulence levels are obtained [26]. Conventionally, swirl has been used in two valve SI engines and diesel engines, while tumble has been preferential for four valve engines due to valve cylinder head symmetry aspects. Swirl generation is rather difficult at such conditions without deteriorating flow performance. The use of such in-cylinder flow motion is of major importance for lean burn engines, where the laminar speed is lowered and the flow field has more time to distort the flame until the end of combustion [27]. Also, the flow field directly affects the in-cylinder heat transfer, and as swirl is maintained during the combustion process, extreme fluid motion may decrease engine overall efficiency[23], [28]. The use of early intake valve closure (EIVC) strategy has shown to promote an initial increase in tumble motion near BDC. If the flow motion is not strong enough, the tumble structure may breakdown in the middle of the compression stroke generating lower turbulence levels than the conventional throttled operation [14] [16]. In the other hand, the use of late intake valve closure (LIVC) is expected to maintain similar turbulence levels or even increase them compared to a conventional intake valve closure timing [29]. Lately, with the availability of various valve train solutions such as simpler cam phasing mechanisms to fully variable valve trains, the use of EIVC and LIVC concepts have become more usual. Several systems still use the throttle in order to facilitate load control and only a small number have full valve timing and lift capability. Even then, there is still the question regarding which strategy would be the best in a fully variable valve train scenario for a naturally aspirated engine. Thus, the objective of this work was to identify which of the load controlling strategies through intake valve closure (LIVC or EIVC) result in better fuel economy for unthrottled stoichiometric SI operation with ethanol at low engine speeds. The investigation was focused on the gas exchange process and its effects on combustion and engine out emissions. As the test engine had fully variable capability, the influence of the maximum intake valve lift was also investigated to evaluate its effect on engine operation for the best load control strategy.