Engine capacity used to be the most important engine parameter that defined performance. But that’s no longer the case. We investigate Volvo’s new Drive-E engines, currently at the forefront of the downsizing movement
What’s happened to engine-capacity badging on modern vehicles? The once proud 3.0 is nowhere to be seen. Even the German manufacturers have partaken in the Big Lie: a BMW 328i sports a 2,0-litre, four-cylinder unit. Measuring up an opponent at the lights for an impromptu drag race has become increasingly difficult due to misleading or absent engine-size badging. The culprit? Forced induction…
The amount of torque (and power when engine speed enters the equation) that an internal-combustion engine can produce is mostly influenced by the amount of air that can flow through it. The more air (oxygen) that is available for combustion, the more fuel can be burned. This is why carmakers employ optimised air intakes, multiple valves per cylinder – including variable valve timing and lift – and optimised exhaust systems. Tuners have latched on to the fact and offer induction kits, high-lift camshafts and free-flow exhaust systems for the same reason.
In naturally aspirated engines, capacity plays the biggest role in the amount of torque the engine can produce; everyone knows the popular saying, “There’s no replacement for displacement.” This all changed with forced induction in the form of turbo- and supercharging in today’s production engines. By forcing air into the combustion chambers at higher pressure than atmospheric, more fuel can be burned, which increases the brake-mean-effective pressure (BMEP) in the combustion chamber. Smaller-capacity engines can now easily produce more torque and power than naturally aspirated engines of much larger capacity (for example, the Ford 1,0 Ecoboost engine delivers 92 kW and up to 200 N.m).
After its acquisition by Geely in 2010, Volvo made the decision to develop its own powertrains. Because this is an extremely expensive research and development exercise for a relatively small company such as Volvo, it was important to reduce the engine-layout derivatives across the range. The main goals were fuel efficiency, low emissions and driving pleasure.
The result of this project is a four-cylinder, 2,0-litre layout for both petrol and diesel fuel derivatives featuring downsizing technology that will be implemented on all future Volvos. Even the bore and stroke of the petrol and diesel engines are the same, which enables the company to assemble them on the same engine production line in Skövde, Sweden. Different levels and methods of forced induction ensure a variety of power levels. This power-train layout will replace eight current engine architectures, leading to a drastic reduction in complexity, mass and cost. The architecture also allows for electrification in the form of hybrids in future.
The table below shows the range of power levels planned for the Drive-E four-cylinder engines (info for only the D4, T5 and T6 have been released). The T3, T4, T5 and T6 are turbopetrol engines and the D2, D3, D4 and D5 turbodiesels. The total power envelope spans 88 kW to 225 kW and torque 270 N.m to 480 N.m. You’ll notice the equal bore and stroke of all engines, with the higher-output petrol and diesel engines employing bigger bearing and gudgeon pin sizes owing to the increase in BMEP. This denotes a high commonality between the base engines.
The different methods of forced induction controls the amount of boost pressure over the engine-speed range of each derivative (see Forced induction versus drivability). The peak firing pressure of the petrol engines are knock limited (uncontrollable auto ignition of the air-fuel mixture) and therefore lower than the peak pressures possible in the diesel engines owing to the compression-ignition method of combustion. This explains the torque advantage of the D5 engine over the T6. The fact that the petrol engines can rev higher than the diesel equivalents gives them a power advantage (compare the D5 with the T6).
DOWNSIZING AND OTHER TECH
The term downsizing is commonly used for smaller-capacity engines that use a form of forced induction and direct injection to increase overall engine efficiency. With the Drive-E range, Volvo took the opportunity to employ the following technologies that aid fuel saving and lowering emissions:
• The friction of the piston system was reduced by 50% with a special ring coating, new cylinder-honing specification and lower tangential loads on the piston.
• Using roller bearings and a diamond-like carbon coating on the contacting metal surfaces reduced friction of the camshaft system.
• The oil pressure is fully variable and controlled by a solenoid actuator to supply peak pressure only when needed.
• The engines are designed for low-viscosity oil (0W20) to reduce engine friction.
• Coolant flow can be controlled both electrically in the petrol engines and with a pneumatic control valve in the diesel engines to reduce losses and provide optimal cooling performance.
• A 2 500 bar common-rail injector system with i-ART injectors is employed in the diesel engines. These injectors can measure the local fuel pressure and provide a precise injection quantity more accurately than a single pressure sensor connected to the fuel rail. An added advantage is the more precise adjustment of injector offsets over time to ensure maximum performance over the life of the engine.
Who could have foreseen that a top-of- the-range large SUV such as the next Volvo XC90 will be powered only by a 2,0-litre, four-cylinder engine? Engine capacity and the number of cylinders are becoming meaningless quantities, as engine power and torque are now more a function of forced induction. Petrolheads will bemoan the loss of the soundtrack of six- or eight-cylinder engines (Volvo has had to enhance the engines’ sound quality through the audio speakers), but the fact remains: internal-combustion engines have to evolve or face extinction with the arrival of electric powertrains. The next hypercar may be powered by a 1,0-litre, three-cylinder engine delivering 400 kW.
FORCED INDUCTION VERSUS DRIVABILITY
The problem with forced induction by turbocharging is the loss of drivability. In internal-combustion engines, the difference in airflow through the engine at low speeds and high engine speeds is vast and falls outside the optimal region of a single turbocharger. Variable-nozzle designs and clever wastegate control can extend the ideal boost region, but these steps cannot eradicate turbo lag in highly boosted engines. On the T6 engine, Volvo has employed twin charging by using a belt-driven supercharger below 3 500 r/min to supplement the turbo and eliminate turbo lag. A clutch disconnects the supercharger when boost isn’t needed or when the engine speed passes 3 500 r/min. Because the supercharger is belt driven, it saps power from the engine and leads to a loss of fuel efficiency compared with a turbo that utilises wasted exhaust energy. The improvement in drivability, however, is worth the extra complexity and efficiency loss.