**3. Results**

Two configurations of the engine have been tested: naturally aspirated and turbocharged.

injection parameters (injection strategy, injection timing, and duration).

(CDM) signal that was set to a resolution of 0.1 crank angle degrees.

**Figure 1** shows the engine setup and a detail of the accelerometer location.

heating plug was substituted by the pressure probe).

sumption measurement.

**Table 1.** Engine specifications.

Cylinders 2 Displacement 440 cm<sup>3</sup> Bore 68 mm Stroke 60.6 mm Compression ratio 20:1

84 Improvement Trends for Internal Combustion Engines

Maximum power 8.5 kW @ 4400 rpm Maximum torque 23 Nm @ 2400 rpm

members.

crank angle resolution of the signals.

The engine was installed with an asynchronous motor (Siemens 1PH7, characterized by nominal torque 360 Nm and power 70 kW) in the test bed of Engineering Department at Roma Tre University. The engine was managed by a fully opened ECU, in order to control

HBM T12 was used for torque measurement. AVL Fuel Balance 733 was used for fuel con-

The in‐cylinder pressure was measured with a piezoelectric transducer AVL GU13P (the pre-

The engine speed and the crank angle position were measured by the optical encoder AVL 364C. It generates transistor‐transistor logic (TTL) rectangular pulse signals: one is the trigger signal that was used to compute the engine speed; the other is the code division multiplexing

An Endevco 7240C accelerometer was used to measure the engine block vibration. It is a high‐temperature piezoelectric mono‐axial accelerometer with a nominal sensitivity of 3 pC/g and a resonance frequency of 90 kHz. The vibration signal was conditioned via B&K Nexus device (amplifier and low‐pass filter). A preliminary investigation was devoted to select the optimal position and orientation of the accelerometer, able to guarantee high sensitivity as regards the combustion event and low sensitivity to mechanical sources. Details may be found in Ref. [18]. The accelerometer was mounted on the top of the engine block by means of a threaded pin on one of the stud that fastens the cylinder head to the block. The mounting was chosen in order to ensure a rigid connection to the structural engine

The sampling frequency was varied according to the engine speed, thus to ensure a fixed

This section focuses on the vibration and in‐cylinder pressure data processing and it is devoted to describe in detail the developed methodology. In the first part, some representative crank angle evolutions of in‐cylinder pressure and accelerometer signals related to naturally aspired configuration are shown and results of frequency domain analysis are presented. In the second part, results obtained with the turbocharged engine configuration are shown.

**Figures 2**–**5** present the time‐histories related to naturally aspirated configuration of the engine. The plot of **Figure 2** shows data obtained at 2000 rpm, full load condition with three different injection settings, according to **Table 2**. Case 1 was characterized by two‐shot injections (pre and main injections). In cases 2 and 3, multiple injections (pilot, pre, and main injections) were imposed. These cases differentiate for the injection timings.

**Figure 2.** In‐cylinder pressure and accelerometer signals at 2000 rpm, 100% load.

In‐cylinder pressure traces are superimposed on the corresponding block vibration curves. The accelerometer signals highlight that the mechanical components of the engine vibration (caused by intake and exhaust valve open/close, fuel injection, and piston slap) are less evident than those related to the combustion event.

beginning of the combustion process and agree with the variations in the block vibration traces in the same crank angle interval. **Figure 4** shows how the pressure development and block vibration are affected by a variation of the engine load condition. The contribution of combustion process to the vibration traces is evident: accelerometer signals modify in both

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**Figure 5** shows in‐cylinder pressure in one cylinder and block vibration during one complete engine cycle. The plot highlights that the combustion events in both cylinders affect the accelerometer trend (the combustions have 360 crank angle degrees shift). In the crank angle intervals out of those in which combustion processes take place, a low frequency oscillation is exhibited; the frequency of this oscillation is equal to two times the engine speed value. It is to

time and amplitude accordingly with the variations in pressure traces.

**Figure 3.** In‐cylinder pressure and accelerometer signals at 100% load.

ascribe to engine mechanical components.

**Figure 4.** In‐cylinder pressure and accelerometer signals at 2400 rpm.

The abrupt pressure gradient due to the initial air‐fuel mixture ignition is responsible for high frequency and high amplitude oscillations in the accelerometer traces, regardless of which injection setting is imposed on the engine.

As the injection parameters change, the in‐cylinder pressure development modifies accordingly, and the engine vibration tunes with pressure variations for both the crank angle delay and the maxima amplitude and gradient.

**Figure 3** shows the comparison between signals obtained by imposing on the engine a variation of engine speed. The main differences in the pressure development are located at the


**Table 2.** Injection data at 2000 rpm, 100% load.

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**Figure 3.** In‐cylinder pressure and accelerometer signals at 100% load.

In‐cylinder pressure traces are superimposed on the corresponding block vibration curves. The accelerometer signals highlight that the mechanical components of the engine vibration (caused by intake and exhaust valve open/close, fuel injection, and piston slap) are less evi-

The abrupt pressure gradient due to the initial air‐fuel mixture ignition is responsible for high frequency and high amplitude oscillations in the accelerometer traces, regardless of which

As the injection parameters change, the in‐cylinder pressure development modifies accordingly, and the engine vibration tunes with pressure variations for both the crank angle delay

**Figure 3** shows the comparison between signals obtained by imposing on the engine a variation of engine speed. The main differences in the pressure development are located at the

> **/cycle] SOI [crank angle BthC] pil pre main pil pre main**

Case 1 0 1 13.5 0 16 6 Case 2 1 1 12.5 24 16 6 Case 3 1 1 12.5 22 13 6

dent than those related to the combustion event.

**Figure 2.** In‐cylinder pressure and accelerometer signals at 2000 rpm, 100% load.

injection setting is imposed on the engine.

86 Improvement Trends for Internal Combustion Engines

and the maxima amplitude and gradient.

**Q [mm3**

**Table 2.** Injection data at 2000 rpm, 100% load.

beginning of the combustion process and agree with the variations in the block vibration traces in the same crank angle interval. **Figure 4** shows how the pressure development and block vibration are affected by a variation of the engine load condition. The contribution of combustion process to the vibration traces is evident: accelerometer signals modify in both time and amplitude accordingly with the variations in pressure traces.

**Figure 5** shows in‐cylinder pressure in one cylinder and block vibration during one complete engine cycle. The plot highlights that the combustion events in both cylinders affect the accelerometer trend (the combustions have 360 crank angle degrees shift). In the crank angle intervals out of those in which combustion processes take place, a low frequency oscillation is exhibited; the frequency of this oscillation is equal to two times the engine speed value. It is to ascribe to engine mechanical components.

**Figure 4.** In‐cylinder pressure and accelerometer signals at 2400 rpm.

**Figure 5.** In‐cylinder pressure and accelerometer signals at 3200 rpm, 100% load.

In order to insulate the vibration component mainly related to the combustion development, an analysis of the acquired signals in the frequency domain was performed. Coherence function between in‐cylinder pressure and acceleration signals was computed. Coherence function is defined as the ratio of the cross power spectral density of an input signal (in‐cylinder pressure) and the corresponding output signal (engine block vibration) to the product of the power spectral density of each signal. The function was computed by using windowed data (Hamming window 1/6 of the engine cycle long was used). Further details may be found in Ref. [19].

(approximately in the range 1000–2000 Hz) in which coherence function exhibits the highest values. Tests have been performed in order to investigate the effect of engine operative condition on the relation between in‐cylinder pressures and block vibration signals. From the analysis of the coherence traces obtained in the engine complete operative field, it came out that no matter which the engine operating condition is, it is always possible to select a range of frequency values in which coherence has the highest values thus showing a linear relationship between in‐cylinder pressure and block vibration signals [20, 21]. The processing of the acquired data demonstrated that load condition has a weak effect on the frequency band, whereas it is reliant on the engine speed value, in agreement with results

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For each engine operative condition, the frequency band, in which in‐cylinder pressure and accelerometer traces exhibited high values of correlations, was selected and used to band‐pass filter the vibration data, thus allowing to remove from the signal all the components due to

**Figure 8** shows the obtained filtered accelerometer signal related to 3200 rpm, full load condition. The signal is superimposed on the in‐cylinder pressure trace; both trends were normalized by dividing all data for the maximum amplitude. The plot highlights oscillations of high amplitude in two‐crank angle regions, corresponding to the intervals in which combustion events take place in the cylinders. These oscillations are mainly caused by the combustion since the filtration allowed to keep into the signal only the components highly correlated to

Aimed at relating the combustion process to the filtered accelerometer trace, the rate of heat release (ROHR) was computed starting from the in‐cylinder pressure, through a thermodynamic model in which the Woschni model was used for the instantaneous heat loss to the

obtained during previous experimental activity [20].

sources other than the combustion process.

**Figure 7.** Coherence function at 100% load.

the combustion.

cylinder wall.

**Figures 6** and **7** show the coherence function trends obtained at 2000 and 2400 rpm, full load condition. The plots highlight that it is possible to define a narrow frequency band

**Figure 6.** Coherence function at 2000 rpm, 100% load [20].

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**Figure 7.** Coherence function at 100% load.

In order to insulate the vibration component mainly related to the combustion development, an analysis of the acquired signals in the frequency domain was performed. Coherence function between in‐cylinder pressure and acceleration signals was computed. Coherence function is defined as the ratio of the cross power spectral density of an input signal (in‐cylinder pressure) and the corresponding output signal (engine block vibration) to the product of the power spectral density of each signal. The function was computed by using windowed data (Hamming window 1/6 of the engine cycle long was used). Further details may be found in Ref. [19].

**Figure 5.** In‐cylinder pressure and accelerometer signals at 3200 rpm, 100% load.

88 Improvement Trends for Internal Combustion Engines

**Figure 6.** Coherence function at 2000 rpm, 100% load [20].

**Figures 6** and **7** show the coherence function trends obtained at 2000 and 2400 rpm, full load condition. The plots highlight that it is possible to define a narrow frequency band (approximately in the range 1000–2000 Hz) in which coherence function exhibits the highest values. Tests have been performed in order to investigate the effect of engine operative condition on the relation between in‐cylinder pressures and block vibration signals. From the analysis of the coherence traces obtained in the engine complete operative field, it came out that no matter which the engine operating condition is, it is always possible to select a range of frequency values in which coherence has the highest values thus showing a linear relationship between in‐cylinder pressure and block vibration signals [20, 21]. The processing of the acquired data demonstrated that load condition has a weak effect on the frequency band, whereas it is reliant on the engine speed value, in agreement with results obtained during previous experimental activity [20].

For each engine operative condition, the frequency band, in which in‐cylinder pressure and accelerometer traces exhibited high values of correlations, was selected and used to band‐pass filter the vibration data, thus allowing to remove from the signal all the components due to sources other than the combustion process.

**Figure 8** shows the obtained filtered accelerometer signal related to 3200 rpm, full load condition. The signal is superimposed on the in‐cylinder pressure trace; both trends were normalized by dividing all data for the maximum amplitude. The plot highlights oscillations of high amplitude in two‐crank angle regions, corresponding to the intervals in which combustion events take place in the cylinders. These oscillations are mainly caused by the combustion since the filtration allowed to keep into the signal only the components highly correlated to the combustion.

Aimed at relating the combustion process to the filtered accelerometer trace, the rate of heat release (ROHR) was computed starting from the in‐cylinder pressure, through a thermodynamic model in which the Woschni model was used for the instantaneous heat loss to the cylinder wall.

**Figure 8.** Normalized in‐cylinder pressure and filtered accelerometer signals at 3200 rpm, 100% load.

**Figure 9** shows the ROHR trace superimposed on the accelerometer signal for 3200 rpm, full load condition. Data were normalized with the maximum value. The circle in the plot highlights a zero crossing in the accelerometer trace that indicates the crank angle value corresponding to the start of combustion (SOC).

The same processing was performed with the signals acquired with turbocharged engine configuration. **Figure 11** presents the crank angle evolution of in‐cylinder pressure and acceler-

**Figure 10.** Normalized cumulated sum of rate of heat release and filtered accelerometer trends at 3200 rpm, 100% load.

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The vibration trace appears more noisy in comparison with that one obtained during tests with naturally aspirated configuration (i.e., signals are shown in **Figures 2** and **3**), but the effect of combustion process on the accelerometer signal is evident, as shown in **Figure 12**. In the plot, the vibration signal acquired in fired condition is compared to that one related to the same engine condition but obtained with naturally aspirated configuration. The trace related

ometer trace at 4000 rpm, 100% load.

to motored test at the same value of engine speed is also shown.

**Figure 11.** In‐cylinder pressure and accelerometer signals at 4000 rpm, 100% load (turbocharged).

Starting from the ROHR, the cumulative heat release (CHR) was computed aimed at evaluating the crank angle values corresponding to the burnt mass fraction. **Figure 10** shows the CHR and the filtered accelerometer trend; circles are used to point out SOC and the angular position at which half of the injected fuel is burnt (MFB50).

**Figure 9.** Normalized rate of heat release and filtered accelerometer trends at 3200 rpm, 100% load.

**Figure 10.** Normalized cumulated sum of rate of heat release and filtered accelerometer trends at 3200 rpm, 100% load.

The same processing was performed with the signals acquired with turbocharged engine configuration. **Figure 11** presents the crank angle evolution of in‐cylinder pressure and accelerometer trace at 4000 rpm, 100% load.

**Figure 9** shows the ROHR trace superimposed on the accelerometer signal for 3200 rpm, full load condition. Data were normalized with the maximum value. The circle in the plot highlights a zero crossing in the accelerometer trace that indicates the crank angle value cor-

**Figure 8.** Normalized in‐cylinder pressure and filtered accelerometer signals at 3200 rpm, 100% load.

**Figure 9.** Normalized rate of heat release and filtered accelerometer trends at 3200 rpm, 100% load.

Starting from the ROHR, the cumulative heat release (CHR) was computed aimed at evaluating the crank angle values corresponding to the burnt mass fraction. **Figure 10** shows the CHR and the filtered accelerometer trend; circles are used to point out SOC and the angular

responding to the start of combustion (SOC).

90 Improvement Trends for Internal Combustion Engines

position at which half of the injected fuel is burnt (MFB50).

The vibration trace appears more noisy in comparison with that one obtained during tests with naturally aspirated configuration (i.e., signals are shown in **Figures 2** and **3**), but the effect of combustion process on the accelerometer signal is evident, as shown in **Figure 12**. In the plot, the vibration signal acquired in fired condition is compared to that one related to the same engine condition but obtained with naturally aspirated configuration. The trace related to motored test at the same value of engine speed is also shown.

**Figure 11.** In‐cylinder pressure and accelerometer signals at 4000 rpm, 100% load (turbocharged).

**Figure 12.** Accelerometer signals at 4000 rpm.

An analysis in the frequency domain of the data acquired with turbocharged configuration was performed and the frequency band in which in‐cylinder pressure and accelerometer traces are highly correlated was evaluated. Starting from in‐cylinder pressure data, ROHR, and CHR were computed (they are shown in **Figures 13** and **14**, respectively). In the plots, the data are superimposed on the vibration signal that was band‐pass filtered according to the results of coherence function analysis. Circles are used to highlight in the accelerometer trace the crank angle values corresponding to the SOC and MFB50.

**Figures 15** and **16** show comprehensive plots of results obtained for naturally aspirated and

**Figure 14.** Normalized cumulated sum of rate of heat release and filtered accelerometer trends at 4000 rpm, 100% load

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In each figure, SOC and MFB50 are reported for 3600, 4000, and 4400 rpm, 60, 75, and 100 % load. Data on the *x*‐axis show the crank angle value computed via CHR. Crank angle values

turbocharged engine configuration, respectively.

(turbocharged).

in the *y*‐axis were computed via filtered accelerometer trace.

**Figure 15.** SOC and MFB50 for naturally aspirated engine configuration.

**Figure 13.** Normalized rate of heat release and filtered accelerometer trends at 4000 rpm, 100% load (turbocharged).

**Figure 14.** Normalized cumulated sum of rate of heat release and filtered accelerometer trends at 4000 rpm, 100% load (turbocharged).

**Figures 15** and **16** show comprehensive plots of results obtained for naturally aspirated and turbocharged engine configuration, respectively.

In each figure, SOC and MFB50 are reported for 3600, 4000, and 4400 rpm, 60, 75, and 100 % load. Data on the *x*‐axis show the crank angle value computed via CHR. Crank angle values in the *y*‐axis were computed via filtered accelerometer trace.

**Figure 15.** SOC and MFB50 for naturally aspirated engine configuration.

An analysis in the frequency domain of the data acquired with turbocharged configuration was performed and the frequency band in which in‐cylinder pressure and accelerometer traces are highly correlated was evaluated. Starting from in‐cylinder pressure data, ROHR, and CHR were computed (they are shown in **Figures 13** and **14**, respectively). In the plots, the data are superimposed on the vibration signal that was band‐pass filtered according to the results of coherence function analysis. Circles are used to highlight in the accelerometer trace

**Figure 13.** Normalized rate of heat release and filtered accelerometer trends at 4000 rpm, 100% load (turbocharged).

the crank angle values corresponding to the SOC and MFB50.

**Figure 12.** Accelerometer signals at 4000 rpm.

92 Improvement Trends for Internal Combustion Engines

were used to evaluate indicators able to characterize the combustion development. The angular position of SOC and MFB50 was thus computed via processed accelerometer traces and compared to the same indicators evaluated via the heat release curve. The obtained data highlighted the high reliability of the methodology and indicated its prospective applicability in the real‐ time control of the engine management, in which the control algorithm manages the injection control unit based only on nonintrusive measurement. The comparison between combustion indicators evaluated only by means of the block vibration trend is compared to the optimal values stored in maps previously filled with data for each engine running conditions. The results of

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such a comparison are used as feedback signal to correct the injection settings.

MFB50 Angular position at which half of the injected fuel is burnt

**Nomenclature**

deg Degree

main Main injection

pil Pilot injection pre Pre‐injection Q Injected fuel

R Correlation coefficient

TTL Transistor‐transistor logic

SOI Angular position at which injection starts

SOC Angular position at which combustion starts

Ornella Chiavola\*, Erasmo Recco and Giancarlo Chiatti

\*Address all correspondence to: ornella.chiavola@uniroma3.it Engineering Department, Roma Tre University, Rome, Italy

ROHR Rate of heat release

**Author details**

BTDC Before top dead center

CDM Code division multiplexing

CHR Cumulative heat release

**Figure 16.** SOC and MFB50 for turbocharged engine.

In both plots, the interpolation lines and the corresponding R‐squared values are shown (they are the square of the correlation coefficients).The obtained R values are in all cases very close to the unity, giving a measure of the very high reliability of the relationship between the combustion indicators estimated via accelerometer transducer and computed by direct in‐cylinder pressure measurements.
