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Ricardo report finds hybrid buses have higher regulated emission intensities than conventional buses; need for whole vehicle testing

While technologies for low carbon buses such as hybridization offer the prospect of significant reductions in fuel consumption and CO2 emissions compared to conventionally powered vehicles, the improvement in terms of regulated emissions (criteria pollutants) may not be as great, according to a new report by Ricardo. Local emissions from buses are of particular significance, as the vehicles mainly operate in urban areas.

The UK’s Low Carbon Vehicle Partnership (LowCVP) commissioned the review of the air quality impacts arising from the recent rapid increase in the number of low carbon buses in the UK, a result of subsidies provided through the Government’s Green Bus Funds. The UK now has around 1,300 low carbon buses in operation. The new report recommends that the legislation needs to consider hybrid technology impacts in the test processes to avoid potential unintended consequences in terms of local emissions.

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




If that is the rule "if Lower fuel use,all emissions should reduce proportionatly" , they are asking too much. Reducing any fuel use, emissions or any of these items should suffice.


So, here is what the study wanted to say and did: "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,". The purpose of this statement is to obfuscate and for allowing politicians or other morons to make statements deliterious to alternative clean technologies based on the misleading statements in the study. Sure, they also say that the hybrid vehicles use less fuel, and thus produce lower overall pollution, but the misleading part is there so that opposition to clean technology also gets a way to spin their lie. A simple statement like hybrid buses pollute less is too definitive and doesn't allow for the intentional misreprersentation of facts that the haters of EVs and Hybrids need to stir their minions.

The thing to know is hybrid buses do pollute less than traditional buses. Or, how about hybrid buses pollute less but could be better.

Did they actual have to pay a spin factory to say what they did, when an adult with a brain could have told you the same as what Ricardo produced, and in a more forthright manner.


CNG diesel hybrids should be cleaner, less sulfur and particulates, while using domestic fuel.


Why not use user ICE diesel in hybrid buses and run them continuously at the most efficient cleanest speed?

Alternatively, use an appropriate size FC unit as genset.


@ Brotherkenny4

agreed, this total bullshit.


@ Brotherkenny4

Double agreed.


There is a "most efficient" load and speed point and a “cleanest” load and speed point. Which operating point would you use? Furthermore, if you choose one of these sweet spots for continuous operation, you would have to use a series hybrid system and stop and start the engine to obtain the average power you need. The former (series hybrid) would significantly reduce overall efficiency (compared to parallel or power-split hybrids) and the latter (stop/start, with, in some cases, extended engine-off periods) could increase emissions due to the intermittent operation and related impact on exhaust aftertreatment.

The finding that hybridization could increase emissions (of some emission Components) is nothing new.


Nothing new, and suggesting that slight changes in emissions controls are warranted.  For instance, if excess cooling of catalyst beds increases emissions on engine restart, some combination of superinsulation and electric catalyst heating could address the bulk of it.  Electric heaters could even be used at the bus depot to slash the emissions surge from the first cold start of the day, making life healthier for the workers there.


The ICE stops and starts very often on our Camry HEV and I don't think that it has time to cool off in normal driving, except in serious traffic jams.

If a much smaller ICE was used in a PHEV, it would rarely stop, except when the vehicle is at a stand still for an extended period.


So adding more features to the hybrid buses that increase the cost even more would be the way to overcome this problem. Who would want to buy them then? Perhaps municipalities that do not care about cost, since they use tax payer’s money? Nothing new, I said. Well, not to me, since I have studied this and saw the problem more than a decade ago. For the public, I presume that this article is new information. The general perception is that once you hybridize, emissions will decrease. Now Ricardo has shown that this is not always the case. Immediately people who do not like such information have to come out and label it “BS”. So, perhaps any information you do not like is BS? Boys, grow up!


Governments have mandated far greater expenditures on existing pollution controls.  If a city wants to spend a relatively small amount on fine-tuning of anti-smog gear to make sure their buses are as good neighbors as they can be, who's to tell them they shouldn't?

I saw electrically-heated catalytic converters decades ago.  They had terminals for connections the size of starter-motor cables.  I doubt very much that things have not advanced in the mean time.


Reducing air pollution and noise pollution from city buses should be a mandated objective, even if initial purchase cost is slightly higher. Otherwise, continued decrease in comfort level will happen in many cities. Increase in health care cost and lower general productivity cost will keep going up.

Many large Asian cities are faced with those growing problems and dirty diesel city buses are part of it.

PHEV type city buses with appropriate size FC units as range extenders or pure BEV type may soon be common place solutions.

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