Diesel development

Diesel development

28 Jan 2010

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Aftermarket’s technical consultant Dave Garratt explains how the world of diesel fuels is changing and how to get to grips with the latest 5-gas diesel analysis

TO STUDY diesel engine emissions, we need to look at the burning properties of the fuel. To burn most fuels effectively the ideal air-fuel ratio is 14.7 to 1, which is called ‘lambda 1’. So if all fuels burn at the same ratio, why are diesel engines more efficient?

The diesel engine uses a burning process known as ‘stratified combustion’, which means that at the very point of injection, the fuel burns at about lambda 1.  But because the diesel engine needs excess pressurised air to help self-ignite the fuel, there is a large pocket of pure air surrounding the injected area. It means the overall air-to-fuel ratio is about Lambda 7. This makes the diesel a true lean-burn stratified engine.

Stratified combustion and diesel engines run very clean. However, as with all combustion methods there's a tendency for pockets of air and fuel to burn at slightly richer ratios. As a result, the diesel engine emits small quantities of carbon monoxide and hydrocarbons. Also, due to the high combustion temperatures associated with the stratified combustion process, a diesel will emit significant amounts of NOX.

But that’s not all, when under acceleration, especially at high altitude, the diesel engine burns the fuel that’s directly in the middle of the stratified charge at around lambda 1.3. At this lower air-to-fuel ratio, the pockets of poorly mixed fuel in the cylinder become so rich the engine will emit carbon particulates, which are sometimes seen as soot. Modern emission control methods help to reduce these unwanted emissions. For example, to remove the NOX, diesel engines use one of two processes; Exhaust Gas Re-Circulation (EGR) or Selective Catalyst Reduction (SCR). The small amounts of carbon monoxide and hydrocarbons emitted are dealt with by using a two-way catalytic converter, often called an oxidisation cat.

As for the particulates, one method is to provide more air for combustion by using a turbocharger, especially when at full throttle, under load or at high altitude. With this method, particle emissions can be reduced by 50%.  However, In order to meet the 2005 Euro 4 regulations, diesel particulate filters become common.

Emission control systems

Exhaust Gas Recirculation (EGR)
The EGR system directs a proportion of the exhaust gases back into the inlet manifold via an EGR valve. The gases supplement the combustion air, which reduces the oxygen content and lowers combustion temperatures to reduce NOX emissions. The EGR valve is normally operated by vacuum, supplied from the engine-driven vacuum pump. The vacuum supplied to the EGR valve is regulated by an electrically activated control valve. It’s generally controlled by the engine control unit. Signals from the engine's air-mass meter are constantly monitored by the control unit, which calculates when to implement EGR.   

Selective Catalyst Reduction (SCR)
In SCR systems urea vapor is used to reduce NOX and is injected into the exhaust gas stream. It passes over the catalyst and NOX emissions are reduced by around 80 to 90%. The optimum catalyst temperature is normally around 300 to 400 degrees C. Special catalytic converters are used to make an SCR system work, the current options being a vanadium-based catalyst or a catalyst with so-called ‘zeolites’ mixed into the wash-coat.

Urea is known to most truck drivers as ‘AdBlue’, which is the trade name for a solution of urea in de-mineralised water. It’s stored in a separate tank to the fuel and is sprayed into the exhaust gases. Therefore, it is never mixed or added to the fuel.

Diesel Particulate Filters (DPF)
The latest version of a DPF is known as the Constantly Regenerative Trap (CRT). The CRT particulate filter functions on the basis that soot will oxidize in the presence of NO2 at a lower temperature than with oxygen. The device is made up of two chambers, with the oxidation chamber being separate from the soot collection/combustion chamber. The first chamber contains a substrate coated with an oxidation catalyst, which is designed to oxidize a portion of the NO in the exhaust to NO2. The catalyst also converts CO and HC into CO2 and H2O. In the second chamber, the exhaust flows through a particulate filter, where the soot is trapped. The trapped soot is then destroyed by the NO2, which was produced by the catalyst in the first chamber.

Because this CRT device requires no additive, maintenance is minimal. However Johnson Matthey, the manufacturers of CRTs, recommend that the filter element is cleaned approximately every 60,000 miles. Checking backpressure is a good indicator of when cleaning is necessary. CRT-type filters have been sold for use on heavy-duty diesel engines in Europe, North America, and Japan in a broad range of applications.  Many of these engines are manufactured by Volvo / RVI / Mack, Scania, Cummins, Caterpillar, International, and DaimlerChrysler. Because of their simplicity and low maintenance they have also become the most popular choice of DPF for light vehicle applications.

Diesel fuel and its properties
Diesel fuel is known, in the petroleum refining industry, as ‘gas oil’. Gas oil is distilled from crude oil after the more volatile elements used for petrol are removed.  Chemists would refer to diesel fuel as a heavy hydrocarbon. This is because it contains 12 to 16 carbon atoms per molecule, which is far more than in petrol. It is therefore less volatile. There are three grades of petro-diesel fuel. Nevertheless, only the grade with the highest ignition quality is suitable for light vehicle, high-speed diesel engines.
 
The ignition quality of diesel fuel is identified by its cetane number. The higher the cetane number, the better the ignition quality of the fuel. To ascertain the cetane number of a fuel, it's tested against a mixture of cetane and alpha-methyl-naphthalene.  Adding more Cetane to the mixture increases the mixture’s volatility. When the mixture matches the volatility of the fuel, the % of cetane in the mixture is noted. That % is then known as the fuel’s cetane number. The minimum cetane number for fuel used in high-speed diesel engines is 45. However, cetane levels in excess of 50 are desirable for the optimum operation of modern engines.

Until the early 1990s, standards for diesel fuel quality were not particularly stringent. However, with the advent of tougher emission regulations and the environmental concern about acid rain, the sulphur levels in diesel are being reduced. Such concerns have also led to the search for other fuels. For example, as an alternative to petroleum-based diesel oil, research has led to the development of a substitute vegetable oil, known as bio-diesel.

Bio-diesel is normally made from a mixture of rapeseed oil and methanol, which means that it's almost sulphur free. It also offers great potential for the worldwide reduction of carbon dioxide. This is because rapeseed oil is a renewable fuel, meaning it’s extracted from a living plant. Through photosynthesis, the plant absorbs the carbon dioxide from the air. Oil from the plant is then turned into fuel and is then burnt in the engine. This process releases carbon dioxide back into the atmosphere, and the cycle is thus repeated.

Bio-diesel has other advantages too. Research confirms it reduces the emissions of hydrocarbon, carbon monoxide and particulate emissions. On the negative side, fuel economy and engine power are both reduced by about 10%.  However, a 20% blend of bio-diesel with normal petro-diesel oil reduces the power and economy loss to only 2% while maintaining some of the benefits in emissions.

Vehicle tests have shown that bio-diesel works well at temperatures down to -10 degrees. However, to prevent waxing at temperatures below -10 the fuel should be mixed at equal quantities with normal petro-diesel oil.

Measuring diesel emissions
For years the only workshop method available for measuring diesel emissions was to check the amount of smoke being emitted. Today things are changing. For example, a diesel particulate analyser is available and being used by some European testing authorities to check the correct function of the (DPF). Apart from the vehicle’s own On Board Diagnostics (OBD) this is one of the only ways to test a DPF is functioning correctly.

The 5-gas diesel analyser
However, late last year we sure the introduction of the first diesel 5-gas analyser! Looking at petrol engine emissions is common practise with most technicians knowing exactly what the correct readings should be and what is at fault when incorrect. But when using a 5-gas to look at diesel engine emissions, it takes some familiarisation and is a new experience to most technicians.

Please see below data recorded from a Mercedes Sprinter 412D:

Test     Comment                          Co      HC  No2  Co2   O2
1        Cold engine at idle                 0.06    10    89    2.9    16.9

2        Cold engine at 4300 RPM        0.04    9    104    3.6    15.9

3        Hot engine at idle                   0.02    4    190    2.5    17.56

4        Idle with EGR fully open           0.02    4    34    2.4    17.65

5        Idle with EGR not working         0.02    5    180    2.1    18.14

6        Idle with fuel pump advanced   0.03    10    732    2.9    16.9
          by 10 deg

7        Idle with fuel pump advanced   0.03    10    789    2.3    17.76
          by 10 deg – EGR not working

8        Idle with fuel pump retarded     0.03    8    115    2.5    17.52
          by 2 deg

9        Idle with fuel pump retarded     0.03    7    139    2.1    18.19
          by 2 deg – EGR not working   

10      Blocked air cleaner                   0.07    11    33    5.1    13.89

11      Blocked air cleaner –                0.02    8    224    2.7    17.08
          EGR not working

12      Leaking Injector                      0.04    538    57    2.7    17.68

13      Blocked DPF                           0.02    3    93    1.6    18.10

The No2 readings are very sensitive and useful for diagnostics. Interestingly, they prove that the EGR or SCR systems are working, but most surprising is what happens to the No2 reading when the DPF is blocked. This is caused by the internal EGR, which is created by the back pressure generated in the exhaust system. 

It’s early days when it comes to testing diesel emissions at the tail pipe, but it looks as if diesel 5-gas analysis could be a very useful diagnostic tool. For further information on diesel 5-gas diagnostics please contact the GEA by emailing info@gea.co.uk or visiting the website www.gea.co.uk.
 

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