Kermit’s Endura-E Efficiency

T

hrough my investigations using Kermit’s OBD-II Scanner, and further experimentation with how I drive, I have amassed a great deal of Calculated Load data in conjunction with ECU reported speed. By comparing the calculated load figures in conjunction with the speed and gear, and applying some calculations, it is possible to come up with an arbitrary relative efficiency plot.

As a disclaimer, the chart produced below cannot be to scale. It is not intended to show the relative efficiency of Kermit, gear for gear, but instead to show the relative efficiency of the engine in one gear across a range of engine speeds. It just happens to show third, fourth and fifth gear. It is also important to understand that the chart below represents a gross simplification of the matter and whilst Kermit’s fuel consumption results (below) appear to illustrate that these plots are reasonably accurate, there are far too many variables involved to be anywhere near sure.

Calculating the Data

Using the OBD-II Scanner I’ve built up a massive database of statistics encompassing a range driving conditions. The data has been filtered and sorted into steady speed, level road cruising, as much as possible, and then narrowed down to the nearest 100 rpm.

There are some important limitations of these procedures. It is important to realise that there will be small variations in cruising engine speeds, i.e. a cruise of 2,655 rpm is considered to be the same as a cruise at 2,745 rpm. There are also large environmental variables - air temperature, humidity, wind, fuel quality, weight, so on and so forth - these all make a difference (the weather’s impact is discussed below).

Once I have the data, it is then sorted and processed in one enormous spreadsheet, and broken down into Calculated Load figures for a given series of engine speeds. This process is to eliminate as much as possible all of the “white noise” encountered during the data logging and to produce as reliable as possible data, using a simple arithmetic mean.

After careful analysis, I dropped all results using first and second gear because the data was far too variable. It’s important to note that I very rarely maintain a steady speed in these gears so reported data is relatively sparse. One long haul trip to and from York, using the motorway network and deliberately holding onto a speed in third or fourth gear, is enough to get results that broadly reproduce the data showing below.

As the time of writing (June 2004), the amount of usable data items is only approximately 70,000, from around forty hours of logged data.

The Efficiency Curves – Pre Ecotek Valve

The three curves illustrated are broadly similar, but there are some detail differences between them. However, all three show broadly similar characteristics. At low engine speeds, the dominant characteristic is a high Calculated Load figure for a low engine speed. This is because the engine is labouring simply to maintain a constant speed. Site viewers should note that I have no data for constant speed driving at low engine speeds because this is certainly not kind to the engine.

Click on the image to bring up a larger version.

Peak efficiency, expressed as a modified ratio of Calculated Load and Engine Speed, is achieved when the engine is between 2,350 rpm and 2,550 rpm. In fifth gear this is between speedo reported speeds of 52 mph and 56 mph. The engine achieves a calculated efficiency of greater than 90% between 1,950 rpm and 2,850 rpm in top gear, which corresponds to an indicated 43 mph to 63 mph.

The Weather

The system I use to produce the main charts above is designed to filter out the impact of the weather on the results. However, during one week in late February, the weather was unusually cold and low humidity. How cold? -7°C at sunrise and with no frost - that’s pretty cold. It was cold enough to go out for a drive just to log some data...

Oh and doesn’t the Ka just love cold, dry air?

At a constant speed on the motorway there is very little difference between Calculated Load figures whatever the induction system temperature. With the induction temperature showing -4°C, and there being no wind, the Calculated Load figure was showing 63%.

Results

The three curves are broadly similar. All three rapidly climb from low efficiency (as the engine labours). The taller the gear, the flatter the efficiency curve. In fifth gear, once the engine speed reaches 2,000 rpm, there is a rather more gradual rise until the peak efficiency figure (just under 2,500 rpm), then it gradually falls until it reaches around 2,750 rpm. From this engine speed forwards, the efficiency then drops off rather quicker.

In fourth gear, the plateau from around 2,400 rpm to 2,700 rpm is most prominent. My data for fourth gear is less comprehensive compared with third or fifth, and I believe this is why the chart shows a rather more wobbly line between 1,700 rpm and 2,400 rpm.

As the engine speed rises, so the relative efficiency falls off quite sharply. This is almost certainly because of a greater influenced played by aerodynamics in addition to the Endura-E seemingly being inefficient over 3,000 rpm.

Conclusions

In order to obtain the best fuel consumption figures for a given drive, it would be prudent to keep the engine speed between 1,950 rpm and 2,850 rpm.

Future Action

With Kermit’s standard short final drive ratio, in top gear this means keeping his indicated cruising speed between 43 mph and 63mph.

The Ecotek valve has made some difference – see the article on the valve for further details.

With the taller final drive ratio this would mean keeping the speed between 49 and 72 mph. This should yield important benefits in terms of fuel consumption.

Gear / Speed Considerations

One of the criticisms often levelled at the non-power assisted Ka is that the gearing is rather tall, which means when driving along at 30 in fourth gear, or 40 in fifth gear, the driver has to feather the throttle to avoid labouring the engine. However, from my data, the power assisted model is also very close to labouring at an indicated 30 mph in fourth gear, and an indicated 40 mph in top gear.

This table shows the engine speeds at both 30 and 40 with the different ratios (note that engine speeds are to the nearest 25 rpm and speeds are indicated, not true).

Looking at the table above, we can see that if we’re using the higher gear for that speed, even with the lower final drive unit, the engine revs are still below the 90% efficiency figure of approximately 1,950 rpm.