• Energy Research

AbstractAmmonia (NH3) is emerging as a potential favoured fuel for longer range decarbonised heavy transport, particularly in the marine sector, predominantly due to highly favourable characteristics as an effective hydrogen carrier. This is despite generally unfavourable combustion and toxicity attributes, restricting end use to applications where robust health and safety protocols can always be upheld. In the currently reported work, a spark ignited thermodynamic single cylinder research engine equipped with gasoline direct injection was upgraded to include gaseous ammonia port injection fuelling, with the aim of understanding maximum viable ammonia substitution ratios across the speed-load operating map. The work was conducted at varied effective compression ratios under overall stoichiometric conditions, with the spark timing re-optimised for maximum brake torque at all stable logged sites. The experiments included industry standard measurements of combustion, performance, and engine-out emissions (including NH3 “slip”). With a geometric compression ratio of 11.2:1, it was found possible to run the engine on pure ammonia at low engine speeds (1000-1800 rpm) and loads of 12 bar net IMEP. When progressively dropping down below this load limit an increasing amount of gasoline co-firing was required to avoid engine misfire. When operating at 1800 rpm and 12 bar net IMEP, all emissions of carbon (CO2, CO, unburned hydrocarbons) and NOx decreased considerably when switching to higher NH3 substitution ratios, with NOx reduced by ~ 45% at 1800 rpm/12 bar when switching from pure gasoline to pure NH3 (associated with longer and cooler combustion). By further increasing the geometric compression ratio to 12.4 and reducing the intake camshaft duration for maximum effective compression ratio, it was possible to operate the engine on pure ammonia at much lower loads in a fully warmed up state (e.g., linear low load limit line from 1000 rpm/6 bar net IMEP to 1800 rpm/9 bar net IMEP). Under all conditions, the indicated thermal efficiency of the engine was either equivalent to or slightly higher than that obtained using gasoline-only due to the favourable anti-knock rating of NH3. Ongoing work is concerned with detailed breakdown of individual NOx species together with measuring the impact of hydrogen enrichment across the operating map.

The late intake valve closing (LIVC) load control concept provides a means to eliminate most of the pumping work energy loss. Experimental and theoretical studies have shown that the fuel saving with such a load control system can be as much as 7 per cent. If LIVC is combined with a variable compression ratio (VCR) device, further fuel savings can be realized. Results of up to 20 per cent over a conventional engine, at low loads/speeds have been reported. A simple two-state LIVC control mechanism is currently under development/evaluation at Sheffield Hallam University.

This paper investigates the energy efficiency and emissions benefits possible with connected and autonomous vehicles (CAVs). Such benefits could be instrumental in decarbonising the transport sector. The impact of CAV technology on operation, usage and specification of vehicles for optimised energy efficiency is considered. Energy consumption reductions of 55% - 66% are identified for a fully autonomous road transport system versus the present. 46% is possible for a CAV on today's roads. Smoothing effects and reduced stoppage in the drive cycle achieve a 31% reduction in travel time if speed limits are not reduced. CAV powertrain optimised for different scenarios requires just 10 kW - 40 kW maximum power whilst the vehicle mass is reduced by up to 40% relative to current cars. Urban-optimised powertrain, with only 10 kW - 15 kW maximum power, allows energy consumption reductions of over 71%. UK energy consumption by cars could be 30% - 45% of current levels with a fully autonomous road transport system, depending on an energy efficiency versus travel time trade off. This could be reduced to just 26% if ride-sharing in urban areas achieves a doubling in average occupancy and travel times remain at today's levels. A comparison of IC engine and battery-electric powertrains optimised for a fully autonomous road transport system indicates the benefits of electric powertrain, with a primary energy requirement per unit distance of the equivalent IC engine CAV. Greenhouse gas emissions per unit distance for the battery-electric CAV are 55% of an IC engine CAV with current UK electricity emissions intensity, reducing to 13% at 2030 emissions target levels. Reduced drive cycle energy requirements (44% of current levels) allow greater range and improved economics of electric vehicles whilst reduced power variance allows smaller batteries for hybrids, similarly helping their case.

A first law thermodynamic model has been developed and used to characterize the performance of an automotive engine charge-air intake conditioner system. This system employs a compressor, intercooler, and expander to provide increased charge density with the possibility of reducing, the charge-air temperature below the sink temperature. The model was validated against experimental measurements. The variation of system effectiveness with compressor, intercooler, and expander efficiency was quantified and system operating limits were identified. While the expander was found to have a greater effect than the compressor, the performance of the system was shown to be most dependent upon intercooler effectiveness.