Detection of low Temperature heat release (LTHR) in the Fuel Research (CFR) engine in both SI and HCCI combustion modes

Knock in Spark Ignition (SI) engines remains to be the one of the main barriers in achieving higher efficiency. It is an abnormal combustion phenomena that occurs due to end-gas auto-ignition ahead of the propagating flame. Auto-ignition depends on the pressure-temperature history in the end-gas region and also on the auto-ignition propensity of the fuel, which is related to its composition.

A significant amount of research has been carried out in the last century to understand knock. Fundamental experiments involving measurements of ignition delay in shock tubes, rapid compression machines and chemical kinetic modelling techniques have provided significant understanding of auto-ignition. The key in understanding auto-ignition behavior lies in the oxidation reactions of fuels.

Many of the fuels, especially with n-paraffin content, exhibit reactions before the main high temperature heat release (HTHR). These early reactions are called low temperature heat release (LTHR) and typically occurs at temperatures from 600 to 800 K. The hydrocarbon auto-ignition chemistry can be classified into three different temperature regimes: the low-temperature regime, intermediate-temperature and high-temperature regime.

In a study by Boot et al regarding the auto-ignition of liquid paraffins, the temperature regimes were defined such that: the low-temperature regime was less than 850 K, the intermediate temperature regime was between 850 and 1200 K, and the high-temperature regime was greater than 1200 K.

The auto-ignition process is not only driven by the chemical processes and thermokinetic interactions but highly depends on the low temperature chemistry, which is quite complex and is dominated by the reversible formation of peroxy radicals. The intermediate temperature regime is termed as Negative Temperature Coefficient (NTC) region.

In addition to the NTC region, intermediate temperature heat release (ITHR) is also defined before the main combustion event and the typical temperature range for ITHR is from 850 to 1000 K.

The ranges of the three temperature regimes can vary depending on the fuel type and the operating conditions. One of the early studies to understand the knocking behavior of fuels was conducted by Leppard who demonstrated auto-ignition chemistries for paraffinic, olefinic and aromatic fuels, along with the effects of engine operating conditions, such as intake pressure, temperature and speed on these hydrocarbon components of commercial gasoline.

It was found that the auto-ignition chemistry of paraffinic fuels is dominated by the Negative Temperature Coefficient (NTC) region, whereas for aromatic and olefins, NTC behavior was negligible. A detailed study by Curran et al. on the oxidation of n-heptane and iso-octane was conducted at a range of different pressures, temperatures and equivalence ratios.

The key chemical reactions responsible for fuel oxidation at low and high temperatures were identified and such information was vital in understanding auto-ignition, and hence knock. The initiatives behind choosing n-heptane and iso-octane was due to the fact that both these fuels are used for the octane rating of commercial gasoline.

In addition, the chemical structure of iso-octane was a branched chained hydrocarbon and such type of structure is classified as one of the major chemical classes of gasoline and could play an important role in understanding the fundamental gasoline oxidation. The study on the oxidation reactions of the fuels were not only limited to chemically kinetic modelling, but also the Cooperative Fuel Research (CFR) engine was used to understand the auto-ignition behavior of the fuels. Sahetchian et al conducted experiments in the motored CFR engine to understand the oxidation of n-heptane and n-butane.

The engine was operated at 600 rpm with the spark plug replaced by a fixed microprobe and the compression ratio was varied from 4 to 16 for auto-ignition purposes. The trapped products in the probe were analyzed using thin layer chromatography. By using the same experimental CFR technique, the n-heptane oxidation was further extended to understand the nature of ‘hydro-peroxide complex’ formed during the oxidation of n-heptane.

The anti-knock quality of a fuel is rated by research octane number (RON) and motor octane number (MON) using the standard Cooperative Fuel Research (CFR) engine according to ASTM methods. However, it has been shown in the literature that there are inconsistencies between the knock measured in the CFR engine and modern SI engines.

Szybist et al. conducted a study to understand the knock propensity of the fuels by using both experimental and numerical techniques under modern SI conditions. Fuels with constant RON and varying sensitivity (S = RON-MON) were selected. Combustion analysis provided good insight into the behavior of LTHR and NTC behavior with varying intake temperature and with octane sensitivity. Pre-spark heat release or low temperature heat release (LTHR) was also detected for iso-octane and gasoline from the experiments.

Another study on SI combustion also confirmed the presence of low-temperature heat release where combustion technologies were used to achieve high compression ratios which can lead to high thermal efficiency for SI engine. A 96 RON gasoline was tested experimentally with varying compression ratios from 11.2 to 15 to investigate the drop in the level of torque and the improvement in the fuel consumption.

Alternatively, another combustion mode which has many attractive qualities among the engine community is Homogeneous Charge Compression Ignition (HCCI) due to ultra-low NOx and PM emissions. It has also been found that by studying HCCI combustion, auto-ignition can be understood for the range of pressure-temperature conditions which are not possible in SI engine due to knock.

The HCCI combustion is commonly defined to occur in two stages: low temperature heat release (LTHR) and high temperature heat release (HTHR). It has been found that LTHR plays an important role in HCCI combustion. A small change in the fuel chemical composition or cylinder conditions can lead to changes in the amount of LTHR and its phasing, which can ultimately affect the HTHR.The chemical reactions in the LTHR domain increase the charge temperature inside the cylinder resulting in faster HTHR combustion. In another study a correlation between LTHR with the fuel composition and HCCI combustion was investigated and, by using twelve different fuels, it was found that the chemical structure of hydrocarbons was linked with the LTHR, with n-paraffins resulting in maximum LTHR and the presence of aromatics and olefins reducing the LTHR. It was also found that the HTHR CA50 was a parameter for the indication of combustion knock and was a function of LTHR CA50 and its heating value.

In an HCCI study by Sjoberg et al., it was highlighted that in the presence of LTHR, stable combustion and higher load limits can be achieved which can expand the operating domain of HCCI. It was also shown that the LTHR was quite sensitive to the engine speed. He used a range of fuels with mixtures of iso-octane, n-heptane and toluene and experimentally investigated the effect of engine speed on LTHR. With increasing speed, it was found that LTHR was reduced as less time was available for the low temperature reactions, but intake pressure and EGR acted as a counter to offset the effect of speed and provided control of LTHR and thus allowed expansion of the engine operating regime.

Elsewhere, in another HCCI study, auto-ignition and combustion characteristics for n-heptane and blends of n-heptane/ethanol were studied. It was found that, as the ethanol addition was increased in the ethanol/n-heptane blend, the LTHR was suppressed and initial temperature corresponding to the cool flame reactions increased. In another study by Waqas et al. it was shown that the addition of ethanol into the base fuels lead to a radical scavenging effect and therefore LTHR was reduced. The blends of ethanol/n-heptane were also tested experimentally and numerically under boosted HCCI conditions by Vuilleumier et al..

The focus of that study was to understand the intermediate temperature heat release (ITHR). A good agreement between the experiments and simulations was found and the key reaction pathways contributing to ITHR were identified. Vuilleumier et al. also investigated the fuel composition effect on the heat release characteristics using Primary Reference Fuels (PRFs) under HCCI conditions and found that the magnitude and the duration of LTHR decreased as the octane number of PRFs increased.

This is consistent with the findings of Truedsson et al. where it was shown that maximum LTHR was detected with lower Primary Reference Fuel (PRF) fuel. Vuilleumier et al. showed that the maximum amount of LTHR was observed with highest intake pressure. In summary, there has been a considerable amount of work done in SI and HCCI combustion modes to understand the auto-ignition behavior of gasoline-like fuels.

The full understanding of auto-ignition behavior of the fuels still remains to be a challenge. During the auto-ignition process, the low temperature heat release (LTHR) can provide initial insight into the fuel’s auto-ignition behavior, something which is difficult to capture during propagating flame combustion due to the extensive heat release from the propagating flame.

As highlighted above in the literature, very few studies have been able to measure the presence of LTHR during flame propagation in SI combustion. Currently, standard RON and MON tests using the Cooperative Fuel Research (CFR) engine are used for the octane rating of the fuels and these tests do not provide any information about the LTHR which is known to be present during both octane testing and in modern SI engines. The purpose of this study is to highlight a strategy to detect LTHR in the standard CFR engine. In order to achieve this, the CFR engine was operated under stoichiometric conditions with the engine speed of 600 rpm.

Experiments were conducted under RON-like conditions but with a late spark timing (20 °aTDC) which allowed for LTHR prior to the start of normal spark-driven flame propagation with high enough compression ratio settings. Alternatively, it has been highlighted in the literature that HCCI combustion, which is conducted under a lean environment could be used to understand low and high temperature reactions. There has been a considerable amount of work done in the literature to propose a fuel rating method for HCCI combustion. Recently, it was shown by Trudesson et al. that the Lund-Chevron HCCI fuel number was a useful parameter to describe the fuel auto-ignition behavior in HCCI combustion.

In order to explore if HCCI combustion could be linked with SI auto-ignition behavior, HCCI experiments were also conducted at the same operating conditions as the late spark timing tests. Three fuel blends consisting of iso-octane/n-heptane, toluene/n-heptane and ethanol/n-heptane were tested. The intake pressure and temperature was varied to understand their effect on the compression ratio required to locate the center of the LTHR at top dead center (TDC).

An experimental study using three RON 90 blends was conducted in the standard CFR engine to investigate if it was possible to detect the low temperature heat release (LTHR) of a stoichiometric mixture. Following are the conclusions based on experimental observations:

  1. The use of a late-SI strategy clearly indicated that low temperature reactions are present under stoichiometric conditions, including in the end-gas of SI combustion.
  2. Alternative to late-SI strategy, HCCI combustion also provides a platform to detect and understand low temperature reactions. Qualitatively, the effect of intake pressure and temperature on LTHR was found to be similar between the two combustion modes.
  3. Maximum LTHR could be observed with the boosted intake pressures and low intake temperatures. This was found to be true for both the stoichiometric and lean conditions.
  4. The Intake Valve Closure (IVC) temperature and combustion phasing was found to be critical parameters in determining if HCCI combustion can be linked to understand the LTHR occurrence with late-SI strategy.

References: M.U. Waqas, A. Hoth, C.P. Kolodziej, T. Rockstroh, J.P. Gonzalez, B. Johansson, Detection of low Temperature heat release (LTHR) in the standard Cooperative Fuel Research (CFR) engine in both SI and HCCI combustion modes, Fuel. 256 (2019) 115745. doi:10.1016/j.fuel.2019.115745.

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