The Limitations of Pressure Wave Supercharger: Comparison
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One main direction in improving the efficiency of an internal combustion engine, together with lowering the emissions, is supercharging, i.e., creating a considerable amount of boost for the inlet combustion air. Turbochargers are the most commonly used for this task, but another advantageous alternative exploiting the energy within exhaust gases is the pressure wave supercharger (PWS).

  • pressure wave supercharger
  • emissions

1. Introduction

Our present life is rather difficult to imagine without some facilities that ease our activities, such as computers and their applications, communication devices and, most of all, modern transportation. Their positive impact on our lives and psyches is considerable, but so are their “side effects”. Therefore, scientists search for solutions to indulge these negative effects, especially those with long-term consequences, such as the greenhouse effect on our planet. The main element responsible for this last issue is carbon dioxide, released in a significant amount by the transportation sector. Thus, automotive constructors are interested in producing vehicles with fewer emission propulsion systems, aside from improved performances. Even though there is an ascending trend in releasing electric or hybrid models for most car manufacturers, vehicles equipped with internal combustion engines will keep the top position in the market for many years.
One main direction in improving the efficiency of an internal combustion engine, together with lowering the emissions, is supercharging, i.e., creating a considerable amount of boost for the inlet combustion air. Turbochargers are the most commonly used for this task, but another advantageous alternative exploiting the energy within exhaust gases is the pressure wave supercharger (PWS).
Developed initially in the 1920s’ as a pressure exchanger, the PWS attracted the attention of car manufacturers, mostly during the 1980s’, entering afterwards in a shadow of contention. Lately, a renewed interest was shown by redesigning the classical geometry or by using pressure wave technology in other different applications.

2. Emissions

In the late 1970’s, the Environmental Protection Agency (EPA), by its Emission Control Technology Division, conducted evaluation tests at the EPA Motor Vehicle Emission Laboratory and reported results on tests made on a 220 D Mercedes-Benz engine supercharged with a modified Comprex CX-125 [97][1], compared with a standard 240 D Mercedes-Benz with similar characteristics as the 220 D model. The tests aimed at emissions, fuel consumption and performance. The first advantage of Comprex was the acceleration time of the 220 D-CX over the standard 240 D, which was 18 s from 0 to 60 mph (0–100 km/h) compared to 25.5 s for the 240 D. Exhaust emissions and fuel economy tests were performed according to two procedures: the Federal Test (FTP) and the Highway Fuel Economy Test (HFET). For the emission levels, both cars had similar results [97][1]:
  • The FTP composite results were:
    -
    0.16 g/km HC for 220 D-CX and 0.13 g/km HC for 240 D, lower than the 1978 Federal Statutory Emission Standard (FSES) of 0.25 g/km,
    -
    0.84 g/km CO for 220 D-CX and 0.72 g/km CO for 240 D, lower than the 1978 FSES of 2.1 g/km,
    -
    0.86 g/km NOx for 220 D-CX and 0.96 g/km NOx for 240 D, lower than 1977 FSES of 1.24 g/km but higher than 1978 FSES of 0.25 g/km,
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    252 g/km CO2 for 220 DCX and 254 g/km CO2 for standard 240 D;
  • The HFTP results were as follows:
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    0.07 g/km HC for 220 D-CX and 0.06 g/km HC for 240 D,
    -
    0.47 g/km CO for 220 D-CX and 0.41 g/km CO for 240 D,
    -
    0.75 g/km NOx for 220 D-CX and 0.90 g/km NOx for 240 D,
    -
    184 g/km CO2 for 220 D-CX and 194 g/km CO2 for the standard 240 D.
· The FTP composite results were:
  • 0.16 g/km HC for 220 D-CX and 0.13 g/km HC for 240 D, lower than the 1978 Federal Statutory Emission Standard (FSES) of 0.25 g/km,
  • 0.84 g/km CO for 220 D-CX and 0.72 g/km CO for 240 D, lower than the 1978 FSES of 2.1 g/km,
  • 0.86 g/km NOx for 220 D-CX and 0.96 g/km NOx for 240 D, lower than 1977 FSES of 1.24 g/km but higher than 1978 FSES of 0.25 g/km,
  • 252 g/km CO2 for 220 DCX and 254 g/km CO2 for standard 240 D;
· The HFTP results were as follows:
  • 0.07 g/km HC for 220 D-CX and 0.06 g/km HC for 240 D,
  • 0.47 g/km CO for 220 D-CX and 0.41 g/km CO for 240 D,
  • 0.75 g/km NOx for 220 D-CX and 0.90 g/km NOx for 240 D,
  • 184 g/km CO2 for 220 D-CX and 194 g/km CO2 for the standard 240 D.
The tests showed that for HC and CO, the Comprex supercharged engine has higher emissions, as well as for the CO2 and NOx emissions the 220 D-CX encountered lower values compared to the standard 240D model from Mercedes-Benz.
Additionally, in 1980, the EPA reported results of tests conducted on an Opel Rekord vehicle provided with a 2.3 L diesel engine supercharged with a Comprex CX-112 [98][2]. The maximum power of the engine was 62 kW at 4100 rpm speed. The average measured emissions for gear shifts at 15, 25 and 40 mph (24, 40 and 64 km/h) were (values transformed from g/mile in g/km):
  • According to FTP Mass Emissions: 0.28 g/km HC, 1.025 g/km CO, 206.3 g/km CO2, 0.60 g/km NOx, 0.17 g/km particulates of which ~3.4% sulfate particulates;
  • According to the HFET Mass Emissions procedure: 0.068 g/km HC, 0.36 g/km CO, 244 151.6 g/km CO2, 0.41 g/km NOx, 0.085 g/km particulates of which ~5.2% sulfate particulates.
  • According to FTP Mass Emissions: 0.28 g/km HC, 1.025 g/km CO, 206.3 g/km CO2, 0.60 g/km NOx, 0.17 g/km particulates of which ~3.4% sulfate particulates;
  • According to the HFET Mass Emissions procedure: 0.068 g/km HC, 0.36 g/km CO, 244 151.6 g/km CO2, 0.41 g/km NOx, 0.085 g/km particulates of which ~5.2% sulfate particulates.
These results showed that emissions were lower than the accepted limits of the 1978 or 1982 regulations. Additionally, vehicles presented good total- and part-load response, with smooth acceleration, good driveability and acceptable noise [98][2].
In the early 1990s’, important research on Comprex was conducted at ETH Zurich. Some of them pointed out the emission reduction for engines supercharged with PW Comprex. Amstutz [99][3] proposed using the Comprex as an “exhaust gas recirculation valve”, studying the effects of regulating the gas recirculation rate. Thus, for a manipulated value of the excess air coefficient of 1.9 for a Daimler-Benz diesel engine OM 602, 2.5 L, the Nox emissions were reduced to the value of 65% of the US 83/87 regulated limit value (0.62 g/km), the HC and CO remained lower than the limit values (0.25 g/km HC and 2.1 g/km CO), while the exhaust gas temperature increased, favorable to the catalytic converter, and the particle emissions were 32% lower than the limit value (0.124 g/km), while the consumption remained unchanged.
A comparative study reported in 2009 [88][4] lower emissions of NOx and soot for the diesel engine 493ZQ, 4 cylinders in-line and 17.5 compression ratio supercharged with Comprex CX-102, compared with the same diesel engine classically turbocharged. Lei et al. [88][4] indicated significantly lower emissions for the PWS charged engine, with less than 200 ppm Nox emissions, when the turbocharged engine emissions are between 900…1400 ppm, while the soot is 0.5…4 FSN compared to the turbocharged soot emissions, which vary between 4…6 FSN [88][4]. This difference might rely on the EGR effect inside the PWS channels, which decreases the combustion temperature within the engine cylinders, with great influence on NOx production.

3. Noise

Noise is one of the PWS shortcomings, as the conventional Comprex is working at a wide range of speeds, up to 24,000 rpm, which produces the well-known “whistle”, a penetrating type of noise in a narrow frequency band lying in the audible zone. The noise level depends mainly on the number of channels and their sections [11][5]. Lowering this noise was a difficult task; one solution adapted was to break the symmetry of the rotor cells or by using multiple rows of channels [11][5] or uneven channel sections [6]. Additionally, as a modern PWS can be operated in a very narrow speed range, noise damping measures can be applied effectively.
In 1985, Prof. Berchtold [20][7] reported a noise lowering of about 10 dB when using a cast rotor with variable cell widths and a difference of 5 to 15 dB between the rotors with two rows of cells compared to the rotor with a single staggered cells row, depending on the noise frequencies. The peak of noise intensity was about 85 dB at 500 Hz, while lower results were registered at high frequencies (58 dB at 16 kHz).

4. Fuel Consumption

The evaluation tests made in 1975 at the EPA Motor Vehicle Emission Laboratory [97][1] on the 220 D Mercedes-Benz engine supercharged with CX-125 and the standard 240 D Mercedes-Benz reported in terms of fuel consumption better values for the 220 D-CX:
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9.43 L/100 km for 220 D-CX and 9.52 for 240 D as a composite result;
-
8.2 L/100 km for 220 D-CX and 8.7 L/100 km for 240 D for the Highway fuel consumption.
  • 9.43 L/100 km for 220 D-CX and 9.52 for 240 D as a composite result;
  • 8.2 L/100 km for 220 D-CX and 8.7 L/100 km for 240 D for the Highway fuel consumption.
In 1980, tests made on a modified 2.3 L Opel Rekord with Comprex CX-112 [24][8] stressed fuel consumption during driving cycles of 30.0 mpg (7.84 L/100 km) for the City Test and 38.8 mpg (6.06 L/100 km) for the Highway Test (made according to US regulations), illustrating a satisfactory fuel economy.
Studies and research reports in the late 80’s and early 90’s [19,34,35,36,37][9][10][11][12][13] show fuel-specific consumption for engines supercharged with Comprex as follows (Figure 71):
Figure 71.
Comparative specific effective fuel consumption for passenger car engines (
left
) and truck engines (
right)—(1984–1990). Made based on [36,37].
)—(1984–1990).
-
For passenger cars, the lowest value recorded was 270 g/kWh at about 2000 rpm for the Mazda 626 2.0 L engine, or 220 g/kWh at 2000 rpm for the VW fast diesel. It can be seen that turbocharging seems to get lower values at low engine speeds, while at high speeds Comprex has better results;
-
For truck engines, the pressure wave supercharging led to a fuel specific consumption of about 220 g/kWh for the Saurer 10.8 L diesel engine reached at 1250 rpm, or 230 g/kWh at 1500 rpm for the Daimler-Benz 19.1 L diesel engine.
  • For passenger cars, the lowest value recorded was 270 g/kWh at about 2000 rpm for the Mazda 626 2.0 L engine, or 220 g/kWh at 2000 rpm for the VW fast diesel. It can be seen that turbocharging seems to get lower values at low engine speeds, while at high speeds Comprex has better results;
  • For truck engines, the pressure wave supercharging led to a fuel specific consumption of about 220 g/kWh for the Saurer 10.8 L diesel engine reached at 1250 rpm, or 230 g/kWh at 1500 rpm for the Daimler-Benz 19.1 L diesel engine.
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