The heavy-duty diesel engine is a highly efficient energy converter. The trucking industry is highly sensitive to fuel costs, and therefore high efficiency and low fuel consumption have traditionally been valued.
No sudden leaps in engine efficiency can be anticipated as progress takes place in small increments. Key elements in improving efficiency include (Schreier et al., 2014)(Schuckert, 2016):
optimized combustion
increased compression ratio
optimized air handling including advanced turbocharging concepts
variable valve timing
optimized fuel injection
sophisticated engine controls
reduction of friction
reduction of auxiliary power need and electrification of auxiliaries
downsizing in combination with increased mean effective pressure and reduced engine speed
ultimately waste heat recovery
In 2010, US Department of Energy (DOE) launched the SuperTruck initiative to improve heavy-duty truck freight efficiency of Class 8 trucks by 50 % (expressed in a ton-mile per gallon metric, reference year 2009).
Phase II of the SuperTruck program, launched in 2016, aims at least doubling the freight efficiency. Four OEMs participate from the start:
Cummins, Daimler, Navistar and Volvo. Activities cover a multitude of topics, e.g., improving the powertrain, partial electrification, improved aerodynamics and light-weighting (US DOE, 2016). In 2017, also Paccar joined (US DOE, 2017). Improving engine efficiency is a key topic in the program. The main targets of SuperTruck II are:
• a greater than 100 % improvement in vehicle freight efficiency
• demonstration of a minimum 55 % engine brake thermal efficiency (BTE)
• development of cost effective efficiency technologies
Figure 15 shows the steps to reach 55 % engine efficiency. To reach 55 % efficiency, waste heat recovery is needed. At the SuperTruck II Annual Merit review held in June 2020, achievements of 50 % BTE or above were reported. Cummins reported 53.5 % (Dickson and Damon, 2020) and Daimler 52.9 % (Villeneuve and Girbach, 2020). Navistar (Zukouski, 2020), Paccar (Meijer and Grover, 2020) and Volvo (Amar and Li, 2020) all reported BTE of some 50 %.
Partners from across Europe (all in all 30 actors from 13 countries) are joining forces in the green vehicles project LONGRUN (2020 - 2023) to accelerate the path towards a smarter and more sustainable future.
LONGRUN focuses on engine and fuels for heavy-duty engines. The objectives are (LONGRUN, 2020):
• To achieve an internal combustion engine performance which reaches a 50 % target in terms of peak thermal efficiency; After-treatment systems integrated into hybrid powertrains with advanced engines.
Figure 15: Steps towards 55 % engine efficiency (Villeneuve and Girbach, 2020).
• To achieve over 10 % energy saving (tank to wheel (TtW), excluding effects of plug-in hybrids) and correspondent CO2
reduction; Realisation of robust ICE engine technology for use of future fuels (HVO, dual fuel mixtures), to achieve a major (>90%) CO2 reduction well to wheel.
Significant amounts of work have gone into the research and development of so-called low-temperature combustion schemes, e.g., Homogenous Charge Compression Ignition (HCCI), Partially Premixed Compression Ignition (PCCI) and Reactivity Controlled Compression Ignition (RCCI)(Tunér, 2014)(Reitz and Duraisamy, 2015)(Shim et al., 2019).
The primary motivation for developing these new combustion processes is lowering engine out emission levels (mainly NOx and PM) stemming from low combustion temperature and fully or partly pre-mixed charge.
In addition, there are efficiency gains due to the high combustion rate of partly or fully premixed charge combustion in comparison with diffusion type combustion, resulting in lower thermal losses. A study in 2017 demonstrated 7 % lower fuel consumption compared to baseline for a combined RCCI and dual-fuel combustion system, capable of meeting the Euro VI NOx emission level without the need of exhaust after-treatment (García et al., 2017).
However, the alternative low-temperature combustion schemes have not yet been commercialized. Notwithstanding, early cycle pre-injections in common-rail type fuel systems are used to facilitate the combustion process. Figure 16 is a schematic of alternative concepts and innovation in engine development.
Engines designed for a specific mono-molecular fuel, e.g., di-methyl ether (DME), could provide benefits for efficiency as well as emission reductions (IEA AMF, 2020a). Also paraffinic diesel fuel can reduce emissions, particularly PM emissions, and improve efficiency slightly (IEA
AMF, 2017). Tunér of Lund University projects that a Double Compression Expansion (DCEE) Engine running on neat methanol could have the potential of reaching 60 % BTE (Tunér and Verhelst, 2020).
Optimizing engines merely for efficiency is not possible as there is a requirement to meet increasingly stringent emission regulations. Today all US 2007/2010 and Euro VI compliant engines are equipped with wall-flow particulate filters, which actually increase fuel consumption due to increase in back-pressure and also occasional forced increase in exhaust temperatures to facilitate filter regeneration.
As for NOx emissions, US 2004 and Euro V regulations could be met using only exhaust gas recirculation (EGR) for NOx control. One notable disadvantage of EGR was increase in fuel consumption. In the case of US 2007/2010 and Euro VI, all engines have selective catalytic reduction (SCR, urea catalyst) for NOx control. Some manufacturers use SCR only, some a combination of mild EGR and SCR.
Figure 16: Alternative concepts and innovation in engine development (Meijer and Grover, 2020).
Figure 17 shows the emission control system of a Euro VI diesel engine.
As discussed, over the past two decades regulatory requirements for reduced emissions by government agencies has led to considerable advancements in both engine and after-treatment technologies.
As the efficiency of ICEs go up, exhaust temperatures drop, which can be challenging for efficient exhaust after-treatment. Recent developments include cylinder deactivation and late intake valve closing as strategies that can reduce airflow and increase exhaust temperatures in heavy-duty diesel engines. These technologies can help catalyst-based after-treatment systems maintain effective emission control under low load operation, while also reducing fuel consumption(MECA, 2020).
Figure 17: Schematics of the emission control system of a Euro VI diesel engine (Mackaldener, 2014). (DOC= diesel oxidation catalyst, DPF= diesel particulate filter, ASC= ammonia slip catalyst).