Octane Appetite: The Relevance of a Lower Limit to the MON Specification in a Downsized, Highly Boosted DISI Engine

Abstract

Market demand for high performance gasoline vehicles and increasingly strict government emissions regulations are driving the development of highly downsized, boosted direct injection engines. The in-cylinder temperatures and pressures of these emerging technologies tend to no longer adhere to the test conditions defining the RON and MON octane rating scales. This divergence between fuel knock rating methods and fuel performance in modern engines has previously led to the development of an engine and operating condition dependent scaling factor, K, which allows for extrapolation of RON and MON values. Downsized, boosted DISI engines have been generally shown to have negative K-values when knock limited, indicating a preference for fuels of higher sensitivity and challenging the relevance of a lower limit to the MON specification.The Ultraboost engine is an inline-4 downsized, highly boosted prototype DISI engine designed to achieve a 35% reduction in CO 2 emissions whilst maintaining performance of a production V8. A series of 14 fuel formulations were tested to probe engine response to various fuel properties. This paper presents results from a 7 fuel RON and MON decorrelated matrix at four high-load engine conditions. The K-value was found to be negative at all engine conditions; fuels of higher sensitivity were found to yield improved engine performance. Furthermore, in-cylinder experimental data from high load knocking conditions with a single standard octane fuel were used to simulate the K-value; a similar trend between theory and experiment was observed.

Octane requirements of modern downsized boosted gasoline engines

“…If however KLSA is plotted against Eq. 1, then the best fit value is found to be K = -1.1 [12]. This is amongst the most negative K values ever reported, reflecting the extreme levels of charge pressure employed in this engine.…”

Sub-23 nm Particulate Emissions from a Highly Boosted GDI Engine

“…The second phase was a large matrix of experiments, shown in Figure 5 and was determined by looking at the average knock peak of the four cylinders through a standard AVL algorithm 33 .…”

Alcohol fuels for spark-ignition engines: Performance, efficiency and emission effects at mid to high blend rates for binary mixtures and pure components

“…During the main programme, the engine was used for an extensive series of fuel tests, the results of which, for all but the mid-and high-level alcohol blends, are discussed in detail in a series of other publications. [36][37][38] In the work reported here, the alcohol blends shown in Table 2 are tested. The results will be compared to those of conventional 95 and 98 RON gasoline, and with the Ultraboost control 'base fuel'.…”

“…The other test results have been reported elsewhere. [36][37][38] The test points used are shown in Table 3. BMEP: brake mean effective pressure; BTDC: before top dead centre.…”

“…The higher exhaust back pressure of test point D was intended to reduce the knock limit of the engine with respect to point C. For more information on the rationale behind choosing these test conditions, see Remmert and colleagues. [36][37][38] As described above at each point, a spark loop was conducted with the relative AFR held at l = 1 (i.e. the stoichiometric AFR) and the boost conditions were held constant (i.e.…”