Oil Entry via Piston Top Land

In the present work premature and uncontrolled auto-ignition phenomena in boosted spark ignition direct injection (DISI) engines also referred to as „pre-ignition“ or more commonly as „low-speed pre-ignition (LSPI)“ have been investigated. The work focuses on the identification of the fundamental processes triggering LSPI in order to provide a deeper understanding of this irregular combustion mode and enabling the development of mitigation strategies. LSPI represents an irregular combustion phenomenon that is associated with modern high efficiency engine concepts, i. e. downsized DISI engines, posing a serious challenge in SI engine development. The premature onset of combustion, occurring before the spark plug fires, generally provokes severe engine knock. The most interesting feature of LSPI is the fact that it may occur in bursts consisting of several pre-ignition events alternating with regular cycles. In recent years LSPI has become a prominent issue in SI engine development. A considerable research effort has been spent to unfold the underlying mechanisms of this combustion phenomenon driven by the following reasons: First of all LSPI has a fatal impact on engine durability. Moreover, a common hypothesis assumes that the occurrence of LSPI indicates that a critical limit in engine efficiency has been approached. A combined research approach is applied covering theoretical considerations, numerical simulations of auto-ignition processes, three dimensional CFD computations as well as experimental investigations in a production engine. The research project has been carried out in collaboration with the department of engine research (IFKM) at KIT. The present thesis focuses on the numerical investigations of auto-ignition processes under engine relevant conditions based on a detailed description of the chemical kinetics and the molecular transport processes. The CFD simulations as well as the experimental part have been performed at the IFKM. In a first step potential triggers of LSPI have been identified taking into account the fundamental aspects of auto-ignition chemistry and the governing processes of mixture formation inside the combustion chamber. The conditions that enable a premature auto-ignition in the different potential development scenarios have been determined using numerical simulations. In addition, the sensitivity of auto-ignition with respect to the governing parameters has been analyzed. The comparison of the required conditions predicted by the simulation and the evidence obtained from experimental investigations and CFD computations enables an assessment of the potential of the considered mechanism to trigger LSPI. Considering the results obtained by applying the described approach the majority of the potential sources of LSPI were found to be ineffective. Auto-ignition provoked by incandescent solid particles was identified as the most likely agent of LSPI. Such particles are either formed during the incomplete combustion of droplets potentially originating from a liquid reservoir inside the piston top land crevice or due to the flaking of combustion chamber deposits. In addition the hypothesis of a particle driven LSPI occurrence provides a probable explanation for the observed bursts of alternating pre-ignition and regular combustion events. On the basis of the suggested triggering mechanism of LSPI potential mitigation strategies have been derived. Crucial prerequisites for the occurrence of LSPI are the build up of a liquid reservoir in the piston top land gap as well as a rather inefficient gas exchange. The formation of the liquid reservoir may be hindered by adjusting the spray targeting properly and hence reducing spray impingement on the liner. Improving the scavenging reduces the amount of residual gas and decreases the probability of incandescent particles in the compression stroke. The present thesis provides valuable insights in the fundamental mechanisms provoking LSPI in boosted DISI engines and represents a promising basis for further studies in the field of LSPI. The suggested further experimental investigations may provide some of the missing links to promote a fundamental knowledge of LSPI. Moreover, the proposed mitigation strategies may reduce the issues associated with LSPI in production engines.

<div class="htmlview paragraph">The micro-oxidation test, originally developed at Penn State, has previously been used to study the deposit forming tendency of diesel engine lubricants both at Penn State and Caterpillar using deposit levels and aggregate production of sludge precursor high molecular weight polymers to compare the oils. Researchers at General Motors have used the micro-oxidation test to study the kinetics of the oxidation induced deposit forming reactions for certain basestocks and partially formulated lubricants. In this paper, we begin with reporting the procedural refinement of the micro-oxidation test. We then illustrate how we have developed a method based on first order kinetics deposit forming reactions to rank the relative deposit forming tendency of lubricants. We show how the Arrhenius relationship can be applied to the oxidation reactions of fully formulated lubricants at temperatures consistent with those encountered in the piston ring belt area and top land of heavy-duty diesel engines. We then present the correlation we have achieved between the micro-oxidation results and full-scale engine tests for CD, marine, and railroad oils. We conclude with a discussion on the potential for the development of the micro-oxidation test to achieve correlation with other full-scale engine tests and the inherent benefits.</div>

<div class="htmlview paragraph">Unreacted fuel in ring crevice volumes is a significant source of hydrocarbon emissions from spark ignition engines. If the ring crevice is designed with a wider clearance into which the flame will propagate, the unreacted fuel can be reduced. In this paper, the conditions of flame propagation into the ring crevice are investigated by using ionization probes imbedded in the moving piston of a four-stroke spark ignition engine. It is found that the flame propagates into the ring crevice if the quenching distance when the flame reaches the crevice is narrower than the piston top land clearance.</div>

<div class="section abstract"><div class="htmlview paragraph">This work was concerned with study of lubricant introduced directly into the combustion chamber and its effect on pre-ignition and combustion in an optically accessed single-cylinder spark ignition engine. The research engine had been designed to incorporate full bore overhead optical access capable of withstanding peak in-cylinder pressures of up to 150bar. An experiment was designed where a fully formulated synthetic lubricant was deliberately introduced through a specially modified direct fuel injector to target the exhaust area of the bore. Optical imaging was performed via natural light emission, with the events recorded at 6000 frames per second. Two port injected fuels were evaluated including a baseline commercial grade gasoline and low octane gasoline/n-heptane blend. The images revealed the location of deflagration sites consistently initiating from the lubricant itself. With the high octane fuel (and the limited load adopted for safe optical work) lubricant induced pre-ignition was observed, but without knock. This pre-ignition was repeatedly the result of the lubricant deliberately introduced earlier on in the same cycle. With the lower octane fuel, the previously well reported “on-off” knocking nature of pre-ignited knocking combustion was observed during a sequence of cycles following a single injection of lubricant. In addition it was sometimes apparent that cycles with knock would result in oil subsequently being ejected from the piston top land area during the power stroke.</div></div>

Abnormal combustion, in its various forms, has presented a challenge to the design and operation of spark-ignited internal combustion engines for well over a century. During the last decade the relative prevalence of low speed pre-ignition (LSPI) in highly boosted and downsized engines has led to significant research efforts directed at understanding its origins. Two key mechanisms; oil droplet release from the piston top land crevice region and the accumulation and release of in-cylinder deposits, have both been shown to trigger LSPI. With respect to the oil release mechanism, multiple authors have reported a link between LSPI propensity and the calcium treat rate as detergent in the oil additive pack. It is, similarly, widely reported that magnesium based detergent packs do not promote LSPI in the same way. However, no fundamental explanation as to why calcium is ‘bad’ and why magnesium is ‘good’ has yet been presented in the literature. This paper reports the preliminary results from a series of fundamental shock tube experiments exploring the reactions between calcium and magnesium oxides, and a range of gases in relation to their potential as a trigger for LSPI. Visible light emissions are shown from shock heated calcium oxide particles in a 100% carbon dioxide environment at high temperature, T > 1173 K. Unexpectedly, light emitting reactions are also shown for calcium oxide in a 100% nitrogen environment at temperatures T > 773 K, suggesting that there may be a previously unidentified LSPI trigger mechanism in which nitrogen plays a key role. No visible emissions were seen from magnesium oxide powders.

<div class="htmlview paragraph">The temperature and pressure of the charge, reached at the end of the compression stroke when an engine is cranked for starting, decide whether it will start and attain selfsustained running. These, in turn, are affected by the crevice volume in, and the blowby from, the engine, particularly at cold ambient temperatures and low cranking speeds. This paper presents a model to estimate these effects. Tentative values are proposed for the parameters that appear in the model based on experiments performed on small engines motored in a cold chamber. The model can be incorporated in engine cycle simulation programs to allow for crevice and blowby effects.</div> <div class="htmlview paragraph">It is impossible to prevent gas leakage entirely from an operating reciprocating engine. Gas may leak at the valves, the cylinder head gasket, the spark plug gasket, the injector gasket and the piston rings. The gas that leaks from the cylinder past the piston rings into the crankcase is termed “blowby”. Blowby constitutes the major part of the total gas loss from the cylinder [<span class="xref">1</span>] and, unlike the other leakages, cannot be eliminated completely. Gas leakage from an operating engine leads to a decrease in its performance and an increase in hydrocarbon emissions. In an engine that is cranked for starting, gas leakage reduces the temperature and pressure attained at the end of the compression stroke. Gas leakage is particularly severe at low cranking speeds and, in conjunction with the generally poor fuel distribution and preparation prevailing under these conditions, can inhibit cold starting at low ambient temperatures.</div> <div class="htmlview paragraph">There is another feature inherent in the design of the reciprocating engine which has a similar effect on cold starting in that it also reduces the temperature and pressure attained at the end of the compression stroke. This is the unavoidable presence of small crevices in communication with the working space of the cylinder. The working clearances of the engine such as the piston top land clearance, the clearance between the rings and their grooves and ring gaps constitute these crevices. Such crevices could also exist around the inner periphery of the head gasket, spark plug thread clearances, clearance between the fuel injector and its hole, etc. When the cylinder pressure rises, gas is compressed into these crevices where, because of the very large surface/volume ratio, it is cooled rapidly to the metal surface temperature. Thus, in effect, the crevices augment the heat losses from the charge and hamper cold starting. Though much of the crevice gas is returned to the cylinder when the pressure falls, some of it in the ring pack leaks into the crankcase as blowby.</div> <div class="htmlview paragraph">In spite of their importance, the analysis of the effects of gas leakage and crevices on engine performance has not received much attention so far. There have been a few attempts made to analyze blowby in engines [<span class="xref">2</span>,<span class="xref">3</span>]. The performance of the ring pack is influenced by (a) the movements and deflections of the rings under me action of gas pressure, inertia and friction forces, (b) the hydrodynamics of the oil film on the liner, and (c) the finish of the various surfaces moving relative to one another. In view of these complications, it is unlikely that a single comprehensive model that can predict the performance of ring packs of all engines at all operating conditions can be developed. This paper presents a simple model to estimate gas leakage and crevice effects on the engine cycle. Since cold starting at low ambient temperatures is of concern, the model is described as it applies to a motored engine; its extension to an operating engine is straightforward.</div>