Auto-ignition and heat release of alternative engine fuels

• Main Author: Gorbatenko, Inna
• Format: Dissertation
• Language: English
• Published: University of Leeds 2019

Diversification of energy sources and transport decarbonisation are growing concerns of modern societies. Alternative fuels play an important role in addressing these challenges. For the spark ignition (SI) engine, the propensity of the fuel and fuel blends to auto-ignite is a critical characteristic that limits engine efficiency, which can be assessed by the ignition delays (τi). Severity of knock is also dependent upon the duration of heat release rate - the excitation time (τe). In this thesis, detailed evaluations of τi and τe are employed to study the tendency of methane to detonate in comparison with other fuels, employing the detonation peninsula on the ξ/ɛ diagram. The ξ parameter is the ratio of acoustic to auto-ignitive velocity, whereas ε is the ratio of the acoustic wave resistance time in a hot spot to the τe. It is shown that stoichiometric methane/air exhibits very good anti-knock properties in comparison with other fuels under turbocharged engine running conditions. The changes in the auto-ignition behaviour caused by the progressive addition of n-butanol (at 10%, 20%, 40% and 85% vol n-butanol) to gasoline (RON 95, MON 86.6) and its toluene reference fuel (TRF) are studied computationally and experimentally in a rapid compression machine (RCM) under stoichiometric condition at 2 MPa and at 678-916 K. At low temperatures, n-butanol acts as an octane enhancer, reducing low temperature heat release and increasing ignition delays, with marginal additional effects for blends above 40%. This is supported by the results from ξ /ɛ diagram, where higher n-butanol blends lie further away from the developing detonation region. A brute-force sensitivity analysis of the surrogate model suggests that the main reaction inhibiting ignition at low temperatures is H abstraction from the α-site of n-butanol, even for the 10% blend. At higher temperatures, the behaviour reverses as the chain branching routes from H abstraction by OH from the γ-site of n-butanol and from the α-site by HO2 become more dominant, promoting ignition. For the lower blends, the largest discrepancies between simulations and experiments are found in the negative temperature coefficient (NTC) region, where a larger number of reactions contribute to the uncertainty in predicting τi. For the higher blends, the largest discrepancies occur at low temperatures, indicating that uncertainties within the low temperature n-butanol chemistry need to be resolved. Regarding τe, the addition of n-butanol to the TRF blends has a negligible effect. Furthermore, τe, is not influenced by NTC chemistry.