Extensive road tests with
fuel-saving truck silencers

Introduction It is well known that lowered exhaust pressure drop has benefits for an engine. The engine effect increases, and especially, the fuel consumption is reduced. (Silentor diffusortecknology in Scania truck exhaust silencer)   The magnitude of the fuel saving is difficult to establish. Theoretical calculations give a certain, limited fuel-saving effect, larger for turbo-charged engines than for others. Stationary tests in engine test cells show higher numbers. On top of this, there are dynamic only appearing on the road, e.g. faster turbo-charger response and fewer gear shifts with reduced pressure drop.   In order to quantify the fuel-saving effect of a significantly reduced pressure drop, two separate road tests have been carried out on Scania trucks in1999 – 2001.   The investigation was made by Silentor A/S. It was supported financially by the Danish Energy Authority under the EFP programme, project number 1713/99-0003.   Fleet Test In the first test, the fuel consumption of 17 trucks was monitored for two years corresponding to a total of 4 million kilometres. Initially, the trucks were equipped with their original silencers of very high noise attenuation, but also rather high pressure drop. Subsequently, the trucks were fitted with Silentor silencers performing equally well acoustically, but with roughly half the pressure drop.   The fleet test was conducted with kind assistance of three Danish trucking companies: Flemming Nielsen Transport, Spetra and DFDS/Roland Munch. Out of the 17 trucks, 14 were long-haulage trucks typically driven between Denmark and Northern Italy. The engines were 6-cylindered 12-litre engines with 400 hp on average.   In the fleet test, there was a natural variation of several parameters having a considerable influence on the fuel consumption: load, weather, tyres, driving pattern, etc. Therefore, there is a large spread in observed fuel savings. However, the diagram below, showing all data, reveals a significant saving of fuel. The calculated mean value based on 137 months of driving yields a saving of 5 - 6 pct.   Out of the 17 trucks taking part in the investigation, three trucks contributed with extreme observations – either very large savings or actual increases in fuel consumption. If these trucks are taken out, a more conservative average of 2 – 3 pct saving can be calculated.

Fuel savings by reduced exhaust pressure drop in fleet test

Single-Truck Test In the second test, a single truck was run on a specific route 42 times under controlled conditions. The exhaust system was throttled to three different pressure-drop values.   The truck was a 400 hp, 6-cylindered, 12-litre Scania 124. The route was 185 km long and consisted of highway, country-road and suburban stretches. Besides varying the exhaust pressure drop, the truck was run with and without a trailer (40 and 24 tons pay load, respectively) and it  was driven by four different drivers. A data acquisition system monitored velocity, engine speed, pressures and temperatures up- and downstream of the silencer, boost pressure, ambient air temperature, fuel consumption, and GPS co-ordinates.   The resulting data were processed by multiple regression, giving the following effects of changing different parameters. The results were consistent, giving a high level of confidence. The concluded fuel saving of 2 – 3 pct was similar for all four drivers.

Conclusion The two types of experiments gave consistent results. A conservative evaluation of the fleet test and an over-all average of the single-truck test both gave 2 – 3 pct reduced fuel consumption as a result of pressure-drop reduction by one half.   Print report