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Downsizing and Downspeeding of Diesel Engines

Downsizing and downspeeding of diesels are strategies to enhance fuel economy and reduce emissions. They are not yet widespread but could become more common in five plus years. Downsizing and downspeeding strategies have been successfully applied for light duty gasoline vehicles to improve fuel economy and lower emissions and have proven to be effective and reliable. Downsizing (reduction in displacement and cylinder count) of the medium-heavy diesel highway engine is one means to save fuel and thus reduce CO2 emissions, which is especially important in Europe’s regulatory environment.. However, to maintain torque (to keep the freight moving), certain measures will be required: intake air manifold pressure and/or compression ratio will need to rise, which has implications for emissions of NOx. In recent years, some U.S. medium-duty highway diesel engines have actually upsized, not downsized, but that will eventually change. The Cummins ISB series I-6 upgraded from 5.9 to 6.7 liters, as seen in Dodge DOT class 4 truck chassis). Also, the Ford PowerStroke diesel V-8 made by Navistar (seen in DOT class 4 and 5 truck/bus chassis) went from 6.0 to 6.4 liters. Some of the probable fuel consumption increase in the up-sized Cummins ISB was rumored in the literature to be offset by engine downspeeding (see below), although official specs do not reflect that.

Downspeeding is also helpful to improve diesel fuel economy, within reason and where feasible. The urgency of fuel economy has been raised as the U.S. government announced firm plans on August 9, 2011 to regulate fuel economy of heavy duty vehicles for the first time. The new standards will require 20 percent better mpg by model year 2018 for new heavy duty trucks (tractor-trailers). Such heavy rigs now average only 6 mpg. Application of the downspeeding strategy appears to be of rising interest to save fuel, and is mentioned in various industry research reports. Heavy duty truck (US DOT Class 8) diesels are considered medium-speed, with peak power rated at around 2,000 to 2,100 RPM and peak torque at 1,200 RPM. Medium-duty diesels (such as the Cummins 6.7 liter) are slightly higher speed, producing peak power at 3,000 RPM and peak torque at 1,600 RPM.

Engineers have long known about the fuel efficiency benefits of “lugging” engines, pulling heavy loads at lower RPM with high intake manifold pressures. Engine frictional losses (as in bearings) go up with the square of the RPM. So, RPM decreases cascade reductions in friction loads. For example, one half the rotational speed generates only one fourth the friction. This strategy works if the diesel engine can deliver adequate torque at the lower speed to do the job. Theoretically, a slower-turning engine may also enjoy longer life with more time between overhauls (an economic benefit).

To deliver sufficient torque in a slower engine, engine OEMs may need to boost intake manifold pressure, raise compression ratios, or increase displacement—counter to the downsizing/fuel-saving objective. Another means of restoring torque is variable valve actuation (VVA), adjusting lift and/or timing. Variable lift is of special interest. All of these approaches may have emissions implications (pro and con). In the highly regulated world of highway diesels, the mantra is to do nothing that will increase exhaust emissions at the tailpipe, as non-compliant engines can’t be sold.

The downspeeded diesel engine could be re-designed with a longer stroke and air/fuel system adjustments more optimal to the lower RPM operating point. Truck drivers and fleets may reluctantly have to accept reduced peak hp ratings at the lower governed engine speeds—which could slow acceleration (a potential traffic hazard) and render less-effective hill climbing (a drawback in mountainous regions). One solution to reduced diesel power/torque with both downspeeding and downsizing is to convert the powertrain to a diesel-electric hybrid layout. at considerable added cost. That could supplement anemic diesel torque with electric traction motor assist in parallel with the engine or in series with electric-only propulsion boosted by stored energy in batteries charged by a diesel-driven generator. Conventional DOT class 8 diesel city buses list for around $300,000 but the hybrid-electric versions list for around $500,000. Modern turbodiesels, fortunately, produce robust torque over a fairly broad speed range, so the drop off with modest downspeeding should not be too sharp.

Diesels are often “short-shifted” within a small band of RPM as drivers benefit from a high torque rise down low. With a slower-turning engine (lower governed speed), drivers can’t fully rev out, so shifting may need to become more frequent. That is not a problem with an automated manual transmission but a full manual would be more tiring for drivers to operate.

See the following example of an extremely downsized HD diesel engine application (that went from a 12 liter I-6 to 4.8 liter I-4) in a series-type hybrid-electric articulated Mercedes-Benz bus:

There are indications that downspeeding will be a more popular future path then downsizing for medium-heavy diesels, because it is much less disruptive to implement. The fuel economy gains of downspeeding are modest, however: around 7 to 10 percent compared to baseline diesels of today. That improvement alone is insufficient to meet new U.S. mileage standards for heavy trucks but coupled with other fuel efficiency enhancements could help OEMs to meet the new standards.

HD downsized/downspeeded diesel engines are highly stressed. They have to work harder to deliver the same torque and thus have very demanding lubrication oil requirements. In other words, some fuel economy strategies such as downspeeding, introduce new problems for lube oil. The oil may require help to offset accelerated wear. Engine manufacturers may recommend more frequent lube oil changes of these stressed engines. Lubrizol is working on additive formulations (including friction modifiers) that work well with less-viscous engine lube oils--improving fuel economy, in concert with downsizing/downspeeding strategies. The less viscous (lighter) lube oils reduce viscous drag for mated/moving engine parts.