Cleaner Small Diesels Power Light Equipment

Jan. 19, 2015

Last year, we outlined the technology involved in bringing diesel engines into compliance with Tier 4-Final emissions standards and also summarized how various engine manufacturers are applying that technology. This report covers some of the same ground, but focuses on smaller diesels—those with one, two, and three cylinders—that are powering such items as plate compactors, pumps, compressors, boom lifts, generators, horizontal drills, pressure washers, small ride-on rollers, mini excavators, and the like.

These engines typically have less than 60 horsepower. According to Bernhard Schuetzeneder, marketing director for Hatz Diesel, the company’s engines in the one-to-three-cylinder category range from 2 horsepower to around 50, with one-cylinder models topping out at about 14 horsepower, two-cylinder models at around 26, and three-cylinder models ranging from approximately 25 to 50 horsepower. All are air-cooled.

The Kohler Engines diesel lineup, says Jeff Wilke, product manager, ranges from 6.1 to 56 horsepower in this size category, with air-cooled models developing up to 22.1 horsepower. Caterpillar’s two-cylinder C0.5 engine develops up to 13.7 horsepower at 3,600 rpm, and its three-cylinder C0.7, C1.1, and C1.5 provide a range from 16.4 to 37 horsepower; all are liquid-cooled. 

As our earlier report indicated, the application of Tier 4-Final technology can differ not only manufacturer to manufacturer, but also can differ among a particular manufacturer’s various engine platforms, depending on such factors as displacement, intended application, and even  customer expectations.

For example, the engine in a small wheel loader is expected to deliver high torque the instant the throttle is depressed, so the engine manufacturer might choose to pour in all the fuel required to make the instant power, then use a diesel particulate filter (DPF) downstream to catch the resulting excess particulate matter (PM). In a less-demanding application, fuel might be used more conservatively in an effort to better control PM in the combustion chamber, and a simpler diesel oxidation catalyst (DOC) might be all the PM after-treatment required.

At the risk of boring you with yet another review of basic emissions-control technology, we can summarize by saying that, first, technology is out essentially to reduce PM (basically soot) and oxides of nitrogen (NOx), gases toxic upon inhalation; two, technology falls into two broad categories, in-cylinder and after-treatment; and, three, techniques that reduce PM typically encourage NOx formation, and vice versa—the “NOx/PM trade-off.”

In-cylinder techniques might include specific combustion-chamber designs, precise (often electronic) control of fuel injection, high-pressure common-rail fuel systems, advanced in-take air management (sophisticated turbochargers and inter-coolers), retarded injection timing to reduce NOx formation, and exhaust-gas recirculation (EGR), which mixes oxygen-deficient exhaust gases with intake air (either via an external system or simply by changing valve timing) to reduce NOx formation.

After-treatment includes the DOC, a catalyst that deals with such things as carbon monoxide, helps control PM, and sometimes serves to elevate exhaust temperatures. The latter function is important—in some systems, at least—for assisting in burning away (oxidizing) PM trapped in the DPF, which is very effective at trapping PM. If NOx can’t be managed in-cylinder, then a selective catalytic reduction (SCR) system might be used to control it downstream by causing exhaust gases to react with diesel exhaust fluid (basically ammonia) over a catalyst to form elemental nitrogen and water.

A significant challenge for manufacturers of all types (and sizes) of equipment as they install Tier 4-Final engines in their products has been finding under-hood space to accommodate required after-treatment hardware. This packaging problem obviously is compounded when dealing with small equipment.

Small-diesel technology

The U.S. EPA Non-Road and Stationary Emissions Regulations timetable officially required Tier 4-Final compliance at the beginning of 2008 for engines with up to 25 horsepower, and 2013 for engines between 25 and 50 horsepower. New rulings from the EPA in 2013 concerning how these engines are tested for compliance—called Not To Exceed (NTE) and Non-Road Transit Cycle (NRTC) requirements—has to some degree complicated compliance for small-diesel-engine makers.

“The values we had to meet didn’t change,” says Kohler Engines’ Wilke, “but how we had to test to meet those values did change.”

According to Hatz’s Schuetzeneder, the 0-19 kilowatt category (0-25 horsepower), “were particularly impacted because of the 2013 NTE and NRTC changes.”

Despite the challenges, manufacturers of small diesels have moved forward with Tier 4-Final compliance. In some instances, Tier 4-Final compliance for a particular small-displacement engine is limited to a specific horsepower rating (or horsepower range) at a specific engine speed, requiring equipment manufacturers to work closely with engine makers to ensure that the machine is emissions-compliant, yet has adequate power and performance characteristics. The smallest engines in some manufacturers’ lines might be available only for European markets (and other lesser-regulated areas) that do not require less-than-19-kilowatt (25 horsepower) engines to be regulated.

Just as diesels with four or more cylinders might employ a mix of technology to achieve Tier 4-Final compliance, so also do small diesels follow various paths to compliance. That said, in-cylinder techniques seem to predominate, especially in the very smallest displacements. After-treatment is used, however, as evidenced by Caterpillar’s three-cylinder Cat C1.5 that employs both a DOC and DPF. The company’s smaller three-cylinder models, though, the Cat C0.7 and C1.1, handle NOx and PM in-cylinder, as does the two-cylinder C0.5.

“Different strategies are required based on application, size, and horsepower,” says Kohler Engines’ Wilke. “For instance, stand-by emergency generators are exempt from some emissions regulations, and therefore do not require after-treatment, such as DOCs and DPFs.”

Most small-diesel manufacturers seem generally reluctant to provide specifics about their Tier 4-Final technology, perhaps because much of the technology centers on proprietary in-cylinder techniques. A few examples are available, however.

Among Bobcat’s newest engines, for instance, is the three-cylinder, 1.8-liter model, developed with sister company, Doosan Engine Group. The engine uses a procompany Doosan Engine Group and using a proprietary “ultra-low particulate matter combustion” system in concert with a high-pressure/common-rail fuel system, an external EGR system, and a DOC. The engine varies compression pressure with load, says Bobcat, optimizing performance, fuel economy, and emissions control.

“Kohler Diesel air-cooled engines, in some instances, have added internal EGR,” says Wilke, “but with no after-treatment. Liquid-cooled engines in the KDW Series have all been de-rated to meet the less-than-25-horsepower emissions regulations, but continue not to require after-treatment. The KDI Series features a three-cylinder that meets Tier 4-Final standards through the use of high-pressure/common-rail, electronically controlled fuel injection, and specially designed cylinder heads for in-cylinder control. This allows for only the use of a DOC for after-treatment.”

Kubota Engine’s Super Mini Series uses the company’s proprietary Three Vortex Combustion System (E-TVCS). The system uses a spherical “swirl chamber” (positioned above the combustion chamber) into which fuel is initially injected and ignited before entering the main combustion chamber. According to Kubota, “a specially designed piston with a valve recess and a fan-shaped concave produces an optimum air/fuel mixture by generating three intense swirling airflows within the swirl chamber.” E-TVCS, says Kubota, “is aimed at drastic emissions reductions.”

One final point might be worth mentioning: Factors that small-diesel manufacturers must consider in their emissions-reduction strategies are indirect injection (IDI) and air-cooling, design elements not commonly used in larger-displacement engines.

Compared with the direct-injection process, which sprays fuel directly into an engine’s combustion chambers, an IDI system initially sprays (and ignites) fuel into a “pre-combustion” chamber in the cylinder head, and after fuel begins to burn, the charge expands into the main combustion chamber. IDI generally is considered old technology, but in a number of instances, more-advanced, proprietary IDI technology seems to be working effectively in small diesels.

Just how air-cooling might affect the Tier 4-Final strategy that a manufacture chooses to handle emissions seems, again, to be closely held information. Considering, though, that heat management in an engine has a significant affect on the type and volume of pollutants generated, we’re guessing that air-cooling might present a particular challenge when developing methods to control NOx and PM.