Today’s 11–15 liter diesel engines are complex systems that require whole system design optimization. Engine efficiency improvements come in five major categories: heat transfer and exhaust, gas exchange, engine friction, parasitic engine loads and vehicle accessories, and waste heat recovery. Bottoming cycles with efficiency gains on the order of 6–10% capture waste heat at peak engine loads and recycle the energy. These breakthrough systems (or other approaches including electrical or mechanical turbo-compounding) in combination with turbocharging, variable valve actuation, improved combustion chamber design, advanced engine controls, and direct fuel injection systems combine to create an efficient, but expensive, diesel engine.
National Research Council. 2010. Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles. Washington DC: The National Academies Press. link