IFPEN researchers develop grading system to assess diesel engine cold-start performance with different fuels
|Illustration of the main criteria used to qualify a cold start. Credit: ACS, Starck et al. Click to enlarge.|
A team at IFP Energies nouvelles (IFPEN) in France has developed and validated a methodology using a grading system to assess the cold-start performance of a diesel engine with a focus on smoke emissions, taking into account a variety of parameters including start delay, engine stability, and opacity. In a paper in the ACS journal Energy & Fuels, they then report on the use of the methodology to assess the impact of cetane number (CN) and biodiesel content on the cold-operation performance.
Although cold-start performance is often related to spark-ignition engines, they note, the potential increased use of new combustion modes improved with lower compression ratios (CR) (e.g., to values between 14:1 and 16:1) to reduce raw NOx emissions leads to cold-operation performance issues (start delay, idle stability just after start, and smoke opacity) at low temperatures (20 °C and below).
Fuel properties are a key factor in cold operation. The chemical composition, density, volatility, viscosity, and autoignition properties of fuels can have a substantial impact on cold operation. Moreover, the increase in the use of alternative fuels around the world, as in the EU, for example, can have an impact on the properties of fuels. The biodiesel ratio of fuels is increasing worldwide, especially in Europe.
Fatty acid methyl ester (FAME) is commonly referred to as “biodiesel” and is used as a blend component for diesel fuel. Because FAME can differ significantly from conventional fuels in terms of cold-ﬂow properties [cloud point (CP) and viscosity], it seems important to study the impact of biodiesel on the cold-start performance.—Starck et al.
They used several criteria to assess the engine behavior in cold operation:
Starting phase: (i) rail pressure increase delay; (ii) first combustion delay; (iii) speed increase delay; and (iv) exhaust gas opacity during start.
Idle phase: (i) stable idle delay; (ii) engine speed stability after 20 s; (iii) engine speed standard deviation in idle; and (iv) exhaust gas opacity in idle.
On the basis of previous experience in the field of cold starts, it appears that it is difficult to make a global comparison of different tests because a lot of criteria are used to define cold-condition performance and do not vary in the same way. It is for this reason that a rigorous methodology designed to evaluate and compare performance in these conditions has been developed. The idea is to define a grade for each test that is representative of the overall performance of the engine. The higher the grade, the better the cold-start performance of the engine. It is thus possible to choose the optimum setting and make a more rigorous comparison of the performance obtained with different fuels.—Starck et al.
Their chose criteria that demonstrate robustness—i.e., speed increase delay and exhaust gas opacity for the starting phase and stable idle delay, exhaust gas opacity in idle, and engine speed stability after 20 s for the idle phase—to define the grade. For each criterion, they determined a worst value and a best value, corresponding to a worst grade of 0 and a best grade of 10. Between these two values, there is linear interpolation of the grade.
They then weight the criteria to define a grade for the starting phase and the idle phase, then determine a total grade from those two grades.
With this grading system, it is easy to compare cold-start performances of different tests and also to assess behavior during the starting phase and the idle phase.—Starck et al.
They performed engine tests at 25 °C on a common rail, 2 L displacement, four-cylinder diesel engine (Euro 4) with a compression ratio of 16:1, first using the original settings for all of the fuels and then optimizing injection settings for each fuel using a simple methodology.
They tested soybean methyl ester (SME) and rapeseed methyl ester (RME) biodiesels at a blend ratio of 10% with petroleum diesel. They found no significant difference between SME and RME, showing that the structure of the ester is secondary to the cold-start behavior.
Overall, they found that the cetane number has a significant impact on cold-start performance and that the use of cetane improvers does not lead to an acceptable performance for a low cetane fuel. The addition of biodiesel has a negative impact on cold operation by increasing the starting delay and opacity. Nevertheless, they concluded, when the injection settings were simply optimized, the performance obtained in cold conditions with the biodiesel fuel tested can equal those obtained with a conventional fuel.
In their study, the IFPEN team concentrated on the effect of the fuels on smoke, but noted that HC, CO, and NOx emissions are probably modiﬁed as a result of the fuel. To investigate those phenomena in more depth, IFPEN has launched a specific consortium—CODE I (Cold Operation Diesel Emissions Improvement)—the goal of which is to develop measurement methods to assess emissions and the potential for reducing them using specific technologies and strategies.
L. Starck, H. Perrin, B. Walter, and N. Jeuland (2011) Evaluation of Diesel Engine Cold-Start Performance: Definition of a Grading System To Assess the Impact of Fuel. Energy & Fuels DOI: 10.1021/ef200841