Bosch Technology Slashes NOx Emissions with Little Impact on Fuel Economy

FLAT ROCK, MICHIGAN — Bosch has pioneered an emissions reduction technology it says can cut NOx emissions to 10% of current diesel passenger car levels without any additional components. Further, Bosch says the technology can be scaled up for use in medium and heavy-duty diesel engines. The Stuttgart, Germany-based automotive technology supplier made that announcement in April at the company’s annual press conference and provided an update here on Tuesday.

Speaking at the Bosch Mobility Experience press event, Alex Freitag, director of engineering, Bosch powertrain solutions group said the new approach to NOx emissions will keep diesel engines in the game for years to come without adding significant cost to the vehicle.

"The value proposition of the diesel engine is maintained with a minimal impact on fuel economy," he said.

In tests conducted in Germany, a passenger car equipped with a 1.7 liter, 145 hp diesel engine achieved an average NOx output of just 13 mg per kilometer over several test cycles in what Europeans define as an RDE test; that's real driving emissions. The test cycle includes a combination of urban, rural and highway driving.

Since 2017, European legislation has required that new RDE-compliant passenger car models emit no more than 168 milligrams of NOx per kilometer. As of 2020, that limit will be cut to 120 milligrams. While it's difficult to translate these numbers into a useable American emissions regulation comparison, the emission reductions achieved with this new technology are already 10 times lower than limits set to kick in four years from now.

"It's a combination of technologies that made these reductions possible," said Freitag. "We keep NOx low in the cylinder through better air management and injection strategies, while using low- and high-pressure EGR, along with a temperature controlled aftertreatment system using a [diesel oxidation catalyst], [selective catalytic reduction] on the DPF and a downstream SCR with [diesel exhaust fluid] dosing and low-pressure EGR."

According to Freitag, the emissions reduction technology uses no new hardware, just improvements and variations on existing technology.  

The system uses a high-pressure (2,200 bar or 32,000 psi) common rail fuel injection with multiple injection events to shape the heat release rate of combustion. The demands of high-transient cycle urban driving forced Bosch to optimize the turbocharger for all driving conditions and specifically for low-speed high-torque operation (similar to downspeeding). The new software functions allow for better transient control to minimize emissions and reduce the peaks of NOx production associated with aggressive acceleration and stop and go driving.

The technological solution developed by Bosch is a highly responsive air-flow management system for the engine. A dynamic driving style demands an equally dynamic recirculation of exhaust gases. Bosch says this can be achieved with the use of an RDE-optimized turbocharger that reacts more quickly than conventional turbochargers. Thanks to a combination of high- and low-pressure exhaust-gas recirculation, the air-flow management system becomes even more flexible. This means drivers can drive off at speed without a spike in emissions.

The aftertreatment system consists of a diesel oxidation catalyst followed by a combination DPF and SCR devices. There's a DEF injector inline before the DPF/SCR device. Next in line is a low-pressure EGR return from the engine and then a second SCR device with a "clean-up catalyst" to remove any unconsumed ammonia from the SCR process.

Most importantly, the entire aftertreatment system is tightly temperature controlled to optimize its efficiency. To ensure optimum NOx conversion, the exhaust gases must be hotter than 200 degrees Celsius. In urban driving, vehicles frequently fail to reach this temperature. Bosch uses a sophisticated thermal management system for the diesel engine that actively regulates the exhaust-gas temperature, thereby ensuring that the exhaust system stays hot enough to function within a stable temperature range and that emissions remain at a low level.

"The optimum temperature of the entire system is usually 200 degrees Celsius," said Freitag. "We can heat or cool the system with a combination of EGR and post-injection hydrocarbon dosing and a proprietary air management technology on the engine side, not the aftertreatment system itself."

The test on which Bosch returned the astonishing 13 mg per kilometer NOx output was a recognized test procedure using multiple drivers and multiple runs over a prescribed test course in real-world driving, not in a controlled test track environment. When driving in particularly challenging urban conditions, where test parameters are well in excess of legal requirements, the average emissions of the Bosch test vehicles are as low as 40 milligrams per kilometer, Bosch noted in a press release announcing this technology in April.

At that press event in Stuttgart, Bosch allowed dozens of journalists to drive test vehicles equipped with mobile measuring equipment in heavy city traffic, under especially challenging conditions. The results recorded by the journalists were in keeping with the published test results.

Bosch’s new diesel system is based on components that are already available in the market. It is available to customers effective immediately and can be incorporated into production projects.

As for cost and fuel economy, Bosch says it will not make diesel vehicles "any less affordable," nor will it impact diesel's fuel efficiency and thus CO2 output. "As the measures to reduce NOx emissions do not significantly impact consumption, the diesel retains its comparative advantage in terms of fuel economy, CO2 emissions, and therefore climate-friendliness. Diesel will thus remain a climate-friendly option," the company said in a press release.

And Bosch says the technology is scalable.

"This was tested on a 1.7 liter, four-cylinder passenger car diesel engine, but we see no reason why it can't be introduced in larger vehicles," said Freitag.

One of the big hurdles faced by truck and engine makers going into Phase II of the Greenhouse Gas Reduction rules will be the demand for simultaneous cuts to NOx and CO2. It's accepted that fuel mileage usually suffers with efforts to reduce NOx in-cylinder, but this solution may offer opportunities to achieve both, without significant penalties.

Related: Zero Emissions Vehicles Can Still Impact the Environment