NGV Global

Are natural gas vehicles increasing their role as a simpler, more cost competitive, and more elegant solution?

The increasing complexity of diesel engines is creating opportunities for natural gas vehicles

As diesel powered vehicles face increasingly stringent emissions standards of Euro 5/6 in Europe, and EPA 2010 standards in N. America, more complex engine technology and emissions control systems have been adopted. Particulate emissions have been controlled, in the past, with diesel particulate filters (DPF). Now NOx emissions control for the tighter standards frequently requires a combination of advanced engine control, a DPF, and an emissions control system employing selective catalytic reduction (SCR) with a urea solution injected into the exhaust to control NOxemissions. The cost and complexity of the SCR system has raised the question as to whether, in many cases, natural gas vehicles have become a simpler, more cost competitive, and more elegant solution for meeting the stringent emissions standards imposed on light and heavy duty diesel vehicles.

SCR systems have been in Europe for several years, but are about to be launched in North America in the midst of a fierce debate on competing emissions control technologies. The outcome will eventually affect other countries as emissions standards progress to more stringent levels. In order to understand the opportunities and constraints surrounding this debate, we must first understand how the urea SCR system works, and why it is different from past approaches to meeting emissions standards.

Under the Hood

In a step towards lower emissions in North America, a 2007 diesel engine incorporated high pressure fuel injection, exhaust gas recirculation (EGR), a diesel oxidation catalyst (the DOC assists DPF regeneration), and a DPF, and ultra low sulfur diesel fuel. To meet the pending 2010 North American emissions standards, the diesel engine will add an SCR system consisting of a urea tank containing 32.5% urea solution in de-ionized water, urea injection hardware, an SCR catalyst and an ammonia slip catalyst to prevent unwanted ammonia tailpipe emissions. Sensors, including a NOx sensor, provide feedback closed loop control for the DOC model and NOx emissions control, and feed-forward control to the SCR model for urea and ammonia control.

There may also be a urea quality sensor to measure liquid level and quality of the urea as part of the diagnostic system. Heated lines must be provided for the urea tank and injection system, as the urea solution freezes at 11 deg. F. During start-up in <10 deg. F (common in much of N. America in winter), initial NOx emissions will be high until engine coolant melts the urea solution. Copper zeolite has the best low temperature performance for a SCR catalyst, but has potential side effects of dioxin production, and has been prohibited in Japan, and is under review in the US. Iron zeolite is thermally stable, has poor low temperature performance, but is in common use in Europe.

In Europe, the urea solution is labeled AdBlue, and in the US is labeled Diesel Exhaust Fluid, or DEF. Agricultural urea must not be substituted as a cheaper alternative to DEF, since formaldehyde impurities will poison the catalyst, and particles may plug the urea injection system. Urea is used at the rate of about 2% of the diesel fuel consumption, so a 13 L DEF tank on most heavy trucks will provide a range of about 3000 miles per urea fill. DEF infrastructure is expected to be built in the US and Canada at dealerships and truck stops. For light duty diesel vehicles, urea range is expected to be similar to oil change intervals, and will be added at dealerships. In Europe, reports have been made that, in practice, the urea consumption is higher, at up to 5% of diesel fuel consumption.

Driver education is a significant issue with the DEF system to avoid so called “DEFNESS”. There will definitely be a hassle factor, and drivers will now be an active part of ensuring compliance with an emissions standard, to ensure that vehicles will meet in use requirements. Warning systems will be activated at intermediate and low levels of DEF, and if the there is insufficient DEF, driver deterrents will be initiated, normally putting the vehicle into a reduced power, limp home mode. In Europe, apparently 10 starts are permitted after the urea solution runs out, before the truck is disabled.

Cost

There are other issues to consider. For the vehicle manufacturers, system integration is a big task. Space must be found for additional temperature and NOx sensors, urea storage, heated lines, injector, mixer, driver inducement system, and a thawing system. These packaging constraints can be problematical on complicated vehicle layouts. Estimated incremental costs for the system vary considerably, ranging from $4000 – $8000. (Volvo North America has posted a $9600 surcharge for SCR systems on 2010 trucks). The National Truck Equipment Association posts an $8000 surcharge, so these costs are not inconsequential. The cost of the DEF also has to be factored in, and is estimated to be about $2.70 per US gallon, but the price will vary with production cost and commodity pricing.

Use of DEF to control emissions has the advantage that engine efficiency can be increased. Increased engine efficiency produces increased NOx, but now that the SCR system can remove NOx effectively, engine efficiency gains are expected to be in 2 – 3% range in fleet applications. However, this is offset by the need to purchase DEF along with diesel fuel. Since DEF is currently about the same price as diesel in the US, and used at the rate of 2% of the diesel fuel, the net cost per mile may not be much different from pre-2010 engines. In effect, the increased engine efficiency takes care of the cost of the DEF. In Europe, the Ad blue cost is similar, ($2.40/US gal equiv. in the UK). But the diesel is twice the N. American price at about $5.00/US gal equiv., so the cost impact of urea varies from country to country, but is not insignificant). A further advantage with SCR systems arises because the increased efficiency and increased NOx also reduces particulate formation resulting in reduced frequency for active DPF regeneration.

Effects in the Market

Navistar in North America is not using SCR systems in their 2010 engines. Their Maxxforce engines use higher injection pressures, and advanced EGR, without SCR in 2010. However, it is believed that they have accumulated sufficient emissions credits from selling cleaner engines than required prior to 2010, which will allow them to continue with their EGR engines in 2011 and 2012. While their emissions may not meet 2010 limits, they are posted as being cleaner than 2007 standards. Navistar’s view is that familiarity with EGR provides more certainty to build on systems already in use. It offers low cost of ownership, and ease of service, and removes the burden of compliance from the truck owner, who must have a secondary fuel to operate. The development of a true 2010-compliant product will be the test to prove out Navistar’s strategy which runs counter to the SCR strategy being followed by all other diesel engine suppliers in North America. Caterpillar exited the on-highway truck market because technical investments to make their truck engines compliant with 2010 regulations were too expensive.

While the SCR system will work well, with appropriate safeguards against tampering, (it is already working in Europe, albeit after some early teething problems), the cost, complexity, packaging constraints and burden of compliance on the truck operator, appears to be increasing the window of opportunity for heavy duty natural gas engines.

With heavy duty natural gas engines there is no need for DPF and SCR systems, the economics are simpler, the fuel cost is lower, and the life cycle cost of the vehicles is favourable. Because of the low carbon content of natural gas, there are also significantly lower carbon emissions, and the engines are capable of operating on up to 100% renewable biomethane, which provides up to 90% lower GHG emissions than diesel. As a result, engines such as the Cummins Westport ISLG natural gas engine, which has advanced technology bringing its performance close to diesel, has gained market attention from refuse, truck and transit customers. Diesel-like performance and vehicle availability from original equipment manufacturers (OEM) are key drivers for greater natural gas adoption in the heavy truck, refuse and bus segments. Other drivers, such as increasing customer interest in reducing the demand for foreign oil in the US and interest in low carbon fuels to combat climate change are also important factors.

Operational considerations associated with SCR are soon to confront fleet managers. For example, school bus operators in the US, having been subjected to DPF retrofit programs, are balking at further SCR additions to their vehicles, and are leaning to the idea of switching to simpler natural gas fueling.

At the other end of the scale from heavy duty engines, SCR systems will also be applied to light duty diesels in North America. This includes pick up trucks such as the Ford F150. Mercedes is also offering a diesel passenger car, equipped with a BlueTEC SCR emission control system. The DEF will be added by the dealers at the same interval as oil changes. This could result in increased penetration of light duty diesels due to fuel economy advantages over gasoline models. With the growing interest in NGVs in the US and government policy becoming increasingly favorable, this is unlikely to halt the growing momentum of light-duty NGVs.

It is difficult at present to see how all of this will play out in North America, until more experience is gained with heavy duty fleets and the reality of economics, experience, and low carbon fuels set in. However, it does appear that the heavy duty natural gas engines will play an increasing role in future years.

Editor’s note – Readers interested in the cost comparison between 2010 year diesel and natural gas vehicles might be interested in this report from Tiax LLC. The report was originally written in 2005, before the advent of SCR systems and using a ‘Base Case of $25/bbl in 2010, increasing to >$29/bbl in 2022.’ Findings of the report (without SCR considerations) include…

NG vehicles have a price advantage when crude oil reaches a price break- point on an average annual basis. The price break-point is:
– $22/bbl for Refuse Hauler
– $31/bbl for Transit Bus
– $28/bbl for Short Haul Heavy-Duty Truck
NG vehicles could have a significant advantage over diesel when crude oil is in the $60/bbl (2005$) price range on an average annual basis.

The advent of SCR will increase this advantage even further.

Print PDF