To downsize or not to downsize? That’s the engine-development question facing all OEMs. In an initial move to be seen to be more environmentally responsible, for many, downsizing seemed to be a thoroughly sensible answer. Engineers were delighted to demonstrate their ideas for extracting maximum energy from minimum cubic capacity— and marketing executives were equally delighted with the advent of a new angle for product promotion.
But as with most elements of automotive engineering, engine downsizing has proven to be not quite that simple. “Rightsizing” is the alternate term to emerge to describe rather more pragmatic solutions to reducing a vehicle’s carbon footprint. Not as emotive or positive-sounding as downsizing, "rightsizing" nevertheless has found approval among several OEMs and Tier 1 suppliers.
Arguably, “rightsizing” simply means doing it as it’s always been done. For an OEM, the whole point of engine design is to match it to the entire vehicle and sell the result in large numbers at an equally large profit, while meeting fuel-consumption and emissions legislation, not to mention buyers’ expectations.
Mercedes-Benz recently revealed new 3.0-L 6-cylinder engines and even a new V8 (see story here), albeit with capacity reduced by a relatively modest 0.7-L compared to the previous generation. It regards these engines as rightsized for present and anticipated future legislation, environmental responsibility and efficiency requirements, while giving customers what they expect a Mercedes to provide.
Prof. Dr. Thomas Weber, retiring head of Daimler Group Research and Mercedes-Benz Cars development, always talks of “rightsizing” rather than downsizing: “Instead of trimming the number of cylinders from the outset, thereby foregoing refinement and output, there are much more intelligent solutions. Our M176 V8 engine uses cylinder shut off; at part-load up to 3600 rpm it is an especially efficient 4-cylinder. Then, imperceptibly for the vehicle’s occupants, cylinders 2, 3, 5 and 8 cut in.”
Mercedes also is heavily committed to powertrain electrification via 48V technology to enhance efficiency.
At Delphi, Vice President Engineering (Powertrain), Martin Verschoor, believes progress in downsizing won’t slow—but it will change: “So far we have seen very successful downsized engines that are enabled by pressure charging and variable valve control. The benefits on a gasoline engine of keeping the load high and the throttles open are so significant that I can’t see this trend abating, but I can see diverging technical strategies, with different solutions for different types of vehicle.”
While there is a variety of clever solutions proposed by the pressure-charging specialists, he believes that electrification of the powertrain will provide an attractive answer for all but the lowest-cost vehicles: “Testing shows that Delphi’s new 48V electrical system for mild hybrids delivers 50% to 70% of the CO2 and fuel economy benefit of a full hybrid for just 30% of the cost. But the advantages run deeper than that: with a 48V electrical machine, you have enough instant traction energy to fill the low-rpm torque gaps of a highly downsized engine. We see 48V as an enabler for continuing downsizing for the majority of passenger cars, removing the need for some of the additional complexity that could slow progress in this field.”
New control strategies that take advantage of very powerful yet affordable computing are a vital part of 48V systems, he explains. What he terms “clever control” is being used to release further downsizing opportunities through areas like variable valve control and cylinder deactivation techniques that can also be used to enhance Miller-cycle operation, too.
An additional consideration is NOx, which can be more of an issue with aggressively downsized engines. Says Verschoor: “This could lead to some engine designers deciding to increase capacity. “However, for the majority, the benefits of downsizing are so well-established, I can’t see the overall trend changing significantly.”
But there are limits to downsizing, stresses Federal Mogul Powertrain Chief Technology Officer, Gian Maria Olivetti, who recently stated: “We have reached the end of extreme downsizing. It is difficult to imagine a certain level of power with fewer than three cylinders. Also, (Europe's) Real Driving Cycle (RDE) is not helpful for further downsizing because extreme downsized engines—both gasoline and diesel—run at really high loads, so their emissions and fuel economy are compromised.”
Although Federal Mogul Powertrain is developing technologies to reduce friction on all engine sizes, including coatings for pistons, rings and bearings, additional challenges are posed by downsizing, says Olivetti. “More aggressive combustion, higher temperatures and pressures and increasingly corrosive exhaust products place higher demands on many of our components. Stop-start and hybrid operations demand new solutions for running surfaces that are subjected to intermittent operation. To provide the required durability, we are continually increasing the fatigue strength, wear resistance, temperature resistance and corrosion resistance of our products.”
Advances outside the engine
Oil companies also have views on downsizing/rightsizing. Castrol, a subsidiary of global oil company BP, recently launched its advanced NEXCEL oil-change system. Nexcel’s Chief Engineer, Oliver Taylor, said the company is confident of a strong future for ICEs. “The increased loading present in the WLTC (Worldwide harmonized Light Vehicles Test Cycle) and RDE cycles in Europe means that manufacturers will review what the ‘right’ size approach is for modern engines; some will be downsized, some may be bigger. The trend now is rightsizing.
“Engine development is driven by the need to reduce emissions. Increasing the effective engine load level through downsizing reduces the relative magnitude of friction present and, for a spark-ignition engine, reduces the pumping work at part-load. We expect to see increasing levels of sophistication as mild-hybrid control systems mitigate the transient loads on the ICE, enabling tighter control of NOx and PM (particulate matter).”
To help maintain the durability of increasingly heavily loaded components, downsized engines depend on sophisticated oils with complex additives. Taylor stated: “A significant barrier to downsizing is the need to design the engine to accommodate low-performance generic oils later in its life. An electronically connected, smart lubricant management system such as NEXCEL removes that barrier.”
With a sealed oil-management system, engine designers can extend the possibilities available to them in areas like higher average bearing loads and peak temperatures, while retaining durability: “Add the ability to actively manage oil quality through the oil drain interval and you have a powerful new tool that enables new generations of highly-efficient, rightsized engines.”
As recently reported in Automotive Engineering, the UK company Torotrak has developed a CVT-controlled variable supercharger system called V-Charge to make engines downsized to only a liter return performance similar to a 1.5-L, with fuel consumption of that engine significantly reduced.
“For some time, we have raised concerns about the sole use of turbochargers for the increasing levels of boost being employed on downsized engines, both gasoline and diesel,” said Torotrak Group’s Chief Technology Officer, Doug Cross. “Turbocharging creates higher combustion temperatures and pressures that increase the formation of NOx and introduces additional thermal mass into the exhaust stream, which makes it harder to maintain catalyst temperature when running at part-throttle. It is also least effective at matching intake charge delivery against demand when operating at low engine speeds and torque levels that reflect the driving style of owners seeking maximum economy.”
Cross is confident that variable supercharging—instead of turbocharging—in the lower engine speed range (but retaining a larger turbo for higher engine speeds) offers significant advantages, improving control of intake charge delivery while maintaining adequate exhaust temperatures “Combined with aftertreatment using selective catalytic reduction (SCR), this approach enables effective downsizing while meeting lower NOx and particulate targets.”