2020-01-0552 Published 14 Apr 2020 Determination of Octane Index and K in a 2.0L, 4-Cylinder Turbocharged SI Engine Using the Primary Reference Fuel (PRF) Method Siddharth Gopujkar, Jeremy Worm, Joel Duncan, and William Hansley Michigan Technological University Citation: Gopujkar, S., Worm, J., Duncan, J., and Hansley, W., “Determination of Octane Index and K in a 2.0L, 4-Cylinder Turbocharged SI Engine Using the Primary Reference Fuel (PRF) Method,” SAE Technical Paper 2020-01-0552, 2020, doi:10.4271/2020-01-0552. Abstract R esearch Octane Number (RON) and Motor Octane Number (MON) have traditionally been used to describe fuel anti-knock quality. Te test conditions for MON are harsher than those for RON, causing the RON for a particular fuel to be higher than the MON. Previous researchers have proposed the anti-knock performance of a fuel can be described at other operating conditions using the Octane Index (OI), defned as OI=RON-K (RON-MON), where ‘K’ is a weighing factor between RON and MON, and is a function of engine operating condition. Te K-factor indicates that at a particular operating condition, knock tolerance is better described by RON as K approaches a value of 0, and MON as K approaches a value of 1. Previous studies claim that K-factor is dependent only on the engine combustion system and the speed-load point, and that it is independent of fuel chemistry. In most of these studies, K was determined experimentally using linear regression. In this particular study, K was determined using the PRF method for two test fuels; EPA certi fcation tier 2 and tier 3 fuel. K was calculated for these fuels at multiple test points and the results showed that the K was diferent for the two fuels and thus it did depend on fuel chemistry. For a majority of the test points, the fuel with the lower RON and MON values (tier 3 cert fuel) had a lower K-factor as compared to the tier 2 cert fuel. A parameter was developed to relate engine speed, combustion phasing and cylinder trapped mass to predict K-factor, independent of the fuel. Te correlation of K with end gas conditions was also investigated as a part of this project. Introduction K nock is an abnormal combustion phenomenon that occurs in a spark ignition (SI) engine. Te autoignition of the end gas, which is the unburned fuel and air mixture ahead of the advancing fame, leads to the sponta- neous release of chemical energy which causes high pressure oscillations inside the engine cylinder. Te sharp metallic noise made by these oscillations is termed as ‘knock’ [1]. Knock is a hurdle to the betterment of SI engines. An engine subject to knock for a long period can result in structural damage to the engine [2]. Knock constrains a spark ignition engine from performing at its peak efciency at certain points in its speed-load map [3]. In such a case, the engine cannot operate with the spark at Maximum Brake Torque (MBT). Te spark has to be retarded to avoid knock, but in doing so, the operating efciency of the engine is compromised. In such a case, the engine is said to be knock limited [4]. Te spark timing at which the engine is knock limited is known as the knock limited spark advance (KLSA). Te KLSA for a partic- ular engine at a particular operating point depends on the knock mitigating or anti-knock property of the fuel. Te anti-knock quality of fuels used in SI engines is char- acterized by two octane numbers - Research Octane Number (RON) and Motor Octane Number (MON). RON is deter- mined by the ASTM procedure D2699-11 [5] while MON is determined by the ASTM procedure D2700-11 [6]. Te proce- dure involves running a Cooperative Fuel Research (CFR) engine at highly controlled conditions, shown in Table 1. Te operating conditions for the MON test are harsher as compared to those of the RON test, thus the MON rating of a fuel tends to be lower than its RON. In the RON and MON tests, the knocking tendency of a fuel at the test condition is compared with the knocking tendency of primary reference fuels (PRFs) at the same condi- tion. Te RON and MON of a fuel is assigned as the PRF number that matches its knock tendency for the respective test. PRFs are a binary mixture of the straight chain alkane n-heptane and the branched chain alkane iso-octane [7]. Te octane number of iso-octane is 100 by defnition, while that TABLE 1 Test conditions for the RON and MON tests [5, 6] Parameter RON test MON test Engine Speed (RPM) 600 900 Intake Air Temperature (°C) 52 149 Spark Timing (° bTDC) 13 14-26 Coolant Temperature (°C) 100 100 Intake Air Pressure Atmospheric Atmospheric Compression Ratio 4-18 4-18 © SAE International. Downloaded from SAE International by Siddharth Gopujkar, Wednesday, March 25, 2020