The Promise of New Technology
Most engine manufacturers say 2010 will be like turning the clock back to the good old days
In 2002, diesels came with exhaust gas recirculation for the first time.
EGR put more stress on the engine, causing occasional failures of items like head gaskets and making the cooling system work a lot harder. EGR technology meant additional components like EGR valves and coolers and variable-geometry turbochargers that would occasionally fail. And it put a significant damper on fuel economy. Fuel usage increased because backpressure was needed in the exhaust system to drive exhaust back into the engine.
By 2006, the bugs with failed EGR valves and coolers and VG turbos were mostly a thing of the past. Engineers had even strengthened major internal parts to help guarantee durability. They recalibrated the ECM to meet the emissions standard while optimizing fuel economy. Thus, as 2007 approached, engines were running more reliably and had regained some of the lost fuel economy.
But 2007 brought increased levels of EGR to meet a still-lower NOx (oxides of nitrogen) standard and gave us the diesel particulate filter, or DPF, to meet a required significant drop in particulate from levels that already were very low. Lower-sulfur fuel, provided to cut particulate and preserve the DPF, together with the improved CJ-4 engine oil, meant better engine durability but more costly fuel and oil. In spite of increased EGR, engine-tuning tricks yielded fuel economy that was comparable with 2006 in most highway operations. But when the truck spends a lot of time under light load, a lot of fuel is used to regenerate the DPF. Also, at infrequent intervals, ash needs to be cleaned out of the DPF. All in all, not a pretty picture.
So what will 2010 bring? According to experts from most of the engine manufacturers, a breath of fresh air. It will be a lot like turning the clock back to the days before EGR became a factor.
SCR + less EGR = better engines
In 2010, the NOx standard will be an extremely low 0.2 grams per horsepower-hour, about 1/10 of what was allowed in 2004. Meeting this standard is difficult because low NOx is a diesel engine’s toughest emissions challenge. Diesels thrive on heat because it helps burn soot. Reducing NOx in the engine means making an engine that loves to run hot run cooler.
Until now, NOx has been reduced in the cylinder by re-circulating more and more cooled exhaust. Exhaust absorbs a lot of heat from the fuel as it burns, but it also adds heat, soot and acids to the engine and makes it work under greater pressure. Fortunately, there is an ace-in-the-hole called SCR, or selective catalytic reduction. The technology has long been used in power plants and factories to kill the NOx produced in big boilers. Fortunately, it’s practical to adapt it to a heavy-truck diesel.
This requires an SCR catalyst, a device about the size of a DPF and closely resembling an automotive catalytic converter in construction, together with a storage tank, pump, nozzle and microprocessor. A commodity rapidly becoming available at truckstops called DEF, or Diesel Exhaust Fluid, will be pumped into the SCR catalyst at a rate matching the engine’s NOx output. When heated in the catalyst, a chemical called “urea” is formed from the DEF and, as Ed Saxman, product manager for Volvo Powertrain, tells us, “Urea and NOx becomes nitrogen and water. That’s how SCR after-treatment works.”
SCR is so effective it will be quite practical for the 2010 engines themselves to emit more NOx than in 2007. This means the engine design rules will change in a positive direction for engine manufacturers implementing SCR. “Why are we going to use SCR to kill NOx? Because everything else the engine makes can be reduced by making combustion more efficient,” Saxman says. “By adding SCR to EGR, we can meet the standard so handily that we can actually back off the EGR from EPA ’07 levels.”
Gary Parsons, global OEM and industry liason manager at Chevron Oronite Co., LLC, believes the transition to SCR could somewhat resemble the adoption of the catalytic converter in the car market in 1975 because “using SCR increases the number of degrees of freedom in what engine designers can do.” One example of this, he says, is the fact that having some NOx in the engine exhaust actually helps reduce the need to burn fuel to regenerate the DPF.
Naturally, with DEF having to be pumped in on a continuous basis, it’s a commodity like fuel that must be replaced frequently and will cost truck fleets money – about as much per gallon as diesel fuel. Since minimizing the amount of DEF to the predicted 2-5 percent of the amount of fuel will help control the cost of operation, the engine makers will continue with technology that minimizes the NOx coming out of the engine. There will still be EGR, EGR valves, EGR coolers and variable geometry turbos, except in the unusual case of the Detroit Diesel DD15 series of engines (the DD13, -15 and -16 use a turbo-compounding turbine that drives the crankshaft to develop the exhaust system backpressure that forces the exhaust back in, rather than a VG turbo). Exotic fuel-injection systems that divide the traditional single shot of fuel going into several separate shots to reduce NOx also will continue, and Cummins will adopt the XPI common-rail injection system to allow this on their engines for the first time in 2010.
The big difference in 2010 will be that all the exhaust-handling devices won’t be working so hard, and neither will the engine. This will allow the engine to be tuned for fuel economy more effectively than it has been for some time. The end result will be lower running costs, even including the DEF, some engine makers say.