The world’s first naphtha-fueled demonstration passenger car meets driveability targets for idle quality and transient operation, has acceptable noise levels, and achieves Euro 6 emissions compliance.
A ride and drive of the naphtha-fueled vehicle on April 11 in Aachen, Germany, marked a milestone in the development project involving Saudi Aramco and FEV GmbH, an engineering services company. SAE Magazines attended the event—which included a project status report—as an exclusive media representative.
“Naphtha is an example of a less-processed light fuel,” said Amer Ahmad Amer, Chief Technologist for Saudi Aramco’s Fuel Technology R&D Division. The petroleum product requires much less refinery processing and upgrading in comparison to diesel fuel."
The commercial transport sector’s demand for heavy fuels (i.e., diesel and kerosene) is expected to increase dramatically by 2040. That uptick in heavy-fuel consumption will equate to an oversupply of lighter fractions such as naphtha, which occurs in crude oil’s initial distillation.
Such strategic changes in fuel demand are of much interest to Saudi Aramco, the world’s largest global energy provider producing 9.1 million barrels of oil and 9.9 billion standard cubic feet of natural gas daily. Moreover, there is an intrinsic value associated with using naphtha-like lighter fuels in compression-ignition engines.
“With a diesel engine there is not adequate time for in-cylinder mixing of the fuel, so automakers typically upgrade the fuel-injection equipment with higher injection pressures to make sure there is a certain level of fuel/air mixing into the cylinder. What we’re saying is naphtha-like fuels with lower cetane numbers will give you that fuel/air mixing by default,” Amer said.
Unless high-pressure fuel-injection equipment is part of the engine package, current diesel fuels easily ignite when injected into the engine. And this igniting occurs before the fuel is well-mixed with the air in the engine, which leads to the addition of NOx and soot control aftertreatment devices to reduce tailpipe emissions.
“The intent of premix charge compression ignition (PCCI) is to have just enough mixing to minimize soot formation. This lends itself to use less processed and simpler fuels, such as straight run gasoline (SRG) or naphtha. Using less processed fuel in a vehicle is one way to avoid some of the devices for aftertreatment and high injection pressure,” Amer said.
Initial investigations with naphtha fuel were done by Saudi Aramco and FEV researchers using FEV’s High Efficiency Combustion System (HECS) single-cylinder research engine. The research engine ran naphtha fuel at low engine loads and cold engine conditions. According to engine testbed results, the naphtha-fueled single-cylinder engine met or exceeded efficiency and emissions requirements. (More detailed information is available in the SAE paper 2013-01-0267.)
The next phase brought FEV’s HECS demonstrator vehicle, which uses a downsized High Speed Direct Injection (HSDI) 1.6-L four-cylinder diesel engine, into the project.
According to Hans Rohs, Senior Technical Specialist for Passenger Car Diesel Engines at FEV GmbH, the base HSDI engine has a 2000-bar (29000-psi) common-rail injection system with piezo-electric injectors, a two-stage boost system with charge air coolers, a high- and low-pressure EGR system, and closed-loop combustion control. FEV engineers made various modifications so that the engine could operate on naphtha fuel.
“That included increasing the compression ratio from the base of 15:1 to 19:1, which greatly helps in keeping the CO2 and HC emissions low in the lower engine loads,” said Rohs. "And it helps with igniting the naphtha fuel with its lower cetane number. We also installed a glow plug system to help with the ignition during cold engine temperatures."
The engine uses a standard EURO 4 diesel oxidation catalyst (DOC) and a diesel particulate filter. “But we’re not using any NOx aftertreatment,” Rohs said.
New European Driving Cycle (NEDC) vehicle tests were done with three different fuels: diesel, 50 gasoline/50 diesel blend, and naphtha. The demonstration car’s 38 cetane naphtha (vs. the 56 cetane of European diesel fuel) allows more time for the fuel and air to mix before engine combustion begins.
Engine-out NOx levels and tailpipe particulate matter (PM) emissions levels for the naphtha-fueled demonstration vehicle were within EURO 6 levels. “The engine-out PM emission levels with Naphtha fuel were much lower than with the diesel fuel," said Amer. "But the tailpipe HC/CO emissions were slightly higher than EURO 6 levels, so a different DOC is needed.”
The 1590-kg (3505-lb) naphtha-fueled demonstration car also registered 125 g/km CO2. “And that sets a standard for CO2 emissions at that vehicle weight,” Amer noted.