A basic rule of engine operation is, if a technology results in lower fuel burn rate, the air quality emissions on a g/km basis should reduce proportionately. Therefore, AQ [air quality] impact of vehicle technologies should follow the Tailpipe CO₂ estimates given in the main low CO₂ roadmap report.

There are a number of reasons why this may not happen in practice. Example: any technology resulting in periods of engine shutdown in combination with aftertreatment systems relying on achievement/maintenance of light-off temperature. An example of this may be a diesel hybrid bus with SCR because SCR efficiency may be significantly degraded if the exhaust is not hot enough because the engine is off for long periods.

However, this effect will be totally dependent on duty cycle and short periods of engine off in hybrid mode is actually better for the catalyst than idle: operating at idle cools the catalyst due to flow through the catalyst. Engine off only allows radiative cooling, and consequently optimum catalyst operating temperatures are either restored more quickly on restart or are maintained during short idles.

The real world performance of temperature dependent aftertreatment should be validated for technologies such as hybrid or stop-start which significantly change the engine duty cycle.

—“Air Quality Emissions Impacts of Low CO₂ Technology for Buses”

Reviewing worldwide test processes for HGV (heavy goods vehicle) engines, the report says that Euro VI (EPA10 in USA) emissions levels for diesel and gas engines should be broadly indistinguishable (within measurement error) and that levels emitted from both engine types will be very low.

However, the limited whole vehicle test data available to the researchers showed that the real world improvements in regulated emissions often don’t match the expectations from legislative tests due to the operating cycles of engines on the road.

The most robust data available (supplied by Transport for London’s bus fleet) showed that carbon emissions, fuel consumption and local air quality emissions were reduced for the low carbon vehicles compared with conventional buses (chart below).

Hybrid buses make a significant impact on fuel consumption/CO2 emissions per km travelled. The error band (consistency across different models of bus) is much tighter also compared to the conventional buses. In all cases except for HC, the hybrid buses are performing significantly better than conventional buses in terms of absolute air quality emissions in grams per kilometer traveled. However, with the exception of CO, the proportional reduction of AQ emissions is somewhat less than the significant reduction in CO2/fuel burned. Source: Ricardo. Click to enlarge.

However, using an alternative metric of emissions intensity (g/kgCO₂), in some cases the hybrid vehicles showed higher emission levels per unit of fuel burned than conventional buses indicating, researchers said, that there is further scope to optimize emissions control and after-treatment systems around the operating cycle (chart below).

Excepting CO, the emissions intensity (gram per kgCO2 emitted) is higher for the hybrid buses than conventional. I.e., the hybrid buses are doing well at reducing air quality emissions, but not as well as they could. This suggests a significant opportunity for hybrid buses to further improve their absolute emissions performance by reducing emissions intensity via improved powertrain / aftertreatment integration. Source: Ricardo. Click to enlarge.

The researchers recommend, however, that consideration of hybrid technologies in the legislative test cycle is needed to facilitate further air quality reduction. The report also recommends that buses—both conventional and hybridized, and both fossil and alternatively fueled—should be optimized over drive cycles more directly representative of their operational use.

The report also notes that unregulated emissions remain a concern to legislators and are likely to become regulated over time where they are seen to have an air quality impact. Unregulated emissions of current concern are: ammonia, N₂O/NO₂, aldehydes, benzene (from diesels) and methane, ammonia and aldehydes (from gas engines).

The primary focus of our work has been, and will remain, the reduction of carbon and greenhouse gas emissions to mitigate climate change. It’s important, though, that we always keep an eye on other effects of the low carbon shift, including any impacts on local air quality.

Using appropriate test methods for every new technology and application, such as the process we have implemented for the low carbon bus will help us make sure that low carbon is in all ways low emission.

—LowCVP’s managing director, Andy Eastlake

The new research has been published as an additional component of the LowCVP’s report, published in July: “Preparing a low CO₂ technology roadmap for buses.” (Earlier post.)

Ricardo used the following data sources in preparing the report: KBA database for bus engines 2012 (German Federal Transport Authority); US EPA and CARB databases for bus/HD engine certification results; TfL Hybrid bus air quality emissions data provided by LowCVP; TNO report MON-RPT-033-DTS-2009-03840; and Ricardo’s own engine benchmark database and information where not constrained by confidentiality.

Urban air quality is a significant issue for many towns and cities worldwide, alongside wider national and international concerns regarding carbon dioxide emissions. The hybridization of bus fleet powertrains offers a potential means of reducing fuel consumption and CO₂ emissions but as this research highlights, it is important that such low carbon technologies are applied in an optimal manner, taking account of real-world duty cycles in order to ensure that all regulated pollutants—including those of carbon dioxide—are kept as low as possible.

—Dave Greenwood, head of the Ricardo hybrid and electric systems product group

Resources

• Air Quality Emissions Impacts of Low CO₂ Technology for Buses (RD.13/125301.6)