Introduction. p. 1. * corresponding author:

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1 Solid State Electrochemical Pressure Sensor for Lean Direct Injection Engines Jonathan Rheaume *, Wolfgang Hable, Robert Dibble, Albert Pisano University of California at Berkeley March 8 Abstract A solid state sensor located inside the cylinder of a lean direct injection engine provides a signal that is useful for engine control and diagnostics. Our simulation work indicates that an in situ electrochemical sensor responds to variations in temperature and pressure during the compression stroke prior to fuel injection on account of changes in the diffusivity of air. When the temperature of the device is known, a single cell, amperometric oxygen sensor designed for operation in the mass transport limited regime yields a signal that determines pressure. Our laboratory work confirms this observation. * corresponding author: jrheaume@me.berkeley.edu Introduction The higher efficiency of lean engines make them attractive compared to conventional gasoline engines. Recent advances in computer control and in exhaust cleanup address historical problems with diesel engines. No production vehicle to date, however, has a pressure sensor inside each cylinder. The signal from a solid state electrochemical gas sensor located inside the cylinder of a lean direct injection engine enables advanced engine control and diagnostics. During the compression stroke prior to fuel injection, a single cell, amperometric sensor composed of partially stabilized zirconia responds to variations in temperature and pressure. Changes in temperature and pressure affect the diffusivity of oxygen to the cathode, the site of the electrochemical reduction of oxygen. When temperature is known or held constant, a single cell oxygen sensor designed for operation in the mass transport limited regime yields a signal that can be calibrated to indicate pressure during compression. Monitoring the pressure is useful for (1) determination of pilot fuel injection () diagnostic information about each cylinder such as seals, gaskets, valves, blow-by etc., (3) detection of misfire, and (4) calculation of PV-work. We are investigating the adaptation of existing yttria-stabilized zirconia (YSZ) oxygen sensor technology to fabricate a single cell, amperometric, pressure sensor for in situ gas analysis of lean direct injection engines. Amperometric oxygen sensor technology already exists, but not for use inside engine cylinders. Conventional oxygen sensors are neither engineered to withstand the high pressure (1MPa) of combustion, nor were they designed to provide response times fast enough to monitor events in cylinders (in single milliseconds). YSZ has been used for years in oxygen sensors. It has the unique property of conducting oxygen ions (O - ) at high temperature but not electrons. Yttrium substitutes some zirconium atoms in the ceramic electrolyte. The difference in their valences introduces defects into the ceramic lattice. Ions are transported from one vacancy to the next via a hopping mechanism. The yttria also prevents phase changes from one p. 1

2 crystallographic state to the next. Although maximum ionic conductivity occurs at approximately 8% yttria, the high defect concentration weakens the material so partially stabilized zirconia is typically used due to its superior mechanical properties. 1 e - I Single Cell Sensor Analyte Gases E app A O O - O Bulk Gas Diff barrier Cathode: O +4e - O - YSZ electrolyte Anode: O - O + 4e - O Concentration vs. Distance [O ] Figure 1. Cross section view of single cell sensor with oxygen concentration profile. Existing oxygen sensors reside in the exhaust pipe, where they see relatively constant temperature, pressure, and partial pressure of oxygen (Po ). They usually require reference air. The challenge of designing in situ sensors is to make them generate a useful signal over a wide range of dynamically changing conditions (pressure, temperature, oxygen partial pressure, etc.), in ensuring that they survive in a harsh environment, and in making them operate absent of reference air. The mole fraction of oxygen remains constant during compression prior to fuel injection. When the temperature is known, pressure monitoring is possible because the current response is inversely proportional to pressure. Figure 1 shows how a single cell, amperometric sensor functions. Oxygen x diffuses to the cathode, where it adsorbs and dissociates. At the cathode, the monatomic oxygen accepts electrons from the external circuit to form ions. They migrate across the YSZ electrolyte to the anode where they evolve back to gaseous oxygen. In doing so, they relinquish their electrons to the external circuit, which can be measured as current. The current signal depends on diffusion rate of oxygen, which is inversely proportional to pressure for known temperature. The amount of current is proportional to the arrival rate of oxygen at the cathode. The device performs as an oxygen sensor under isobaric conditions such as in the exhaust system where these sensors are conventionally located. Our work indicates that under conditions where the pressure changes, but not temperature or oxygen quantity (such as during the compression stroke of a non-premixed engine), the sensor s signal reveals pressure. This research and development effort is cutting edge in several respects: We are the first to propose mounting a solid state sensor in engine cylinders. We have already performed the first ever simulation of an oxygen sensor operating under dynamic engine conditions. We are the first to examine the use of gel-casting technology from solid oxide fuel cell manufacturing for a sensor with a yttria stabilized zirconia electrolyte. In the following, we discuss our simulation work and present the experimental results that confirm the use of an oxygen sensor to detect pressure. Computer Model Our model calculates gas properties inside a diesel engine at 15º p.

3 intervals of crank angle over the four strokes of the cycle. We made the following assumptions: air standard diesel cycle, ideal gas with constant specific heats, and isentropic compression and expansion. Figure 3 below shows a diesel cycle pressurevolume diagram that we calculated and Figure 4 below shows the pressure versus crank angle according to our simulation. We used the calculated gas properties to determine diffusion to the cathodic pores in the sensor. Figure exhibits the mechanism for mass transport and charge transfer. Upon diffusion to the electrode surface, each oxygen molecule dissociates as it simultaneously adsorbs according to: O (g) O (ads). The adsorbed monatomic oxygen migrates to the triple phase boundary, where gases, noble metal of the electrode, and electrolyte meet. At these sites the oxygen atoms pick up electrons and become solid state ions according to the following charge transfer reaction: O ads) +e - O - (s). The ions then diffuse through the YSZ electrolyte, where they recombine at the anode to evolve gaseous oxygen, losing electrons in the process. O Pore in Cathode Cermet 1. Diffusion. Dissociative Adsorption O ads O ads 3. Surface Diffusion 4. Charge Transfer 5. Ion Conduction Platinum Electrode e - O - YSZ Triple Phase Boundary Figure.Electrode Pore and Charge Transfer Model. determines current. Before the device warms up, however, ion conduction limits output. Conductivity is strongly dependent on temperature. Our model examines both the diffusion and conduction limited current over the cycle. For diffusion, we used Fick s First Law to deduce oxygen flux (corrected for porosity φ and tortuosity τ) as shown below. Φ = D O O τ ϕ dco Equation 1 dx Subsequently we calculated the mass transport limited diffusion current according to: I diff = nfaφ = nfad O O τ ϕ CO Equation L where n is the number of electrons exchanged (in this case, 4); F is Faraday s constant; and A is the surface area of the electrodes. We also examined ionic drift current, which gave us the conduction limited current. We used the following for current density: J = σe = σ(e/l) Equation 3 where σ is conductivity; E is the electric field; E is applied potential; and L is the length of the electrolytic path. We obtained values for conductivity from the literature, and from them calculated a temperature dependent expression for conductivity. The ionic drift current was then calculated as follows: I drift = φσa(e/l) Equation 4 where φ is a function that adjusts the conductivity values for porosity according to DeJonghe. 3 In steady state operation, diffusion is usually the rate limiting step that p. 3

4 Experimental Methods We created single cell amperometric sensors from commercially available potentiometric ones by applying a bias. We purchased potentiometric oxygen sensors, including both a thimble type (Bosch 1376) and a planar type (Bosch 13761). We mounted the sensors in a pressure vessel and applied constant power to their heaters in order to maintain a constant temperature (thimble sensor at 8 C and the planar one at 5 ). We performed voltage sweeps at various pressure levels. We did not exceed 1mV when applying the bias to obviate electrolysis of the ceramic electrolyte. We measured the current response. Results and Discussion A. Simulation We first used our model to simulate the diesel cycle, as shown in the P-V diagram in Figure 3. In our model, we held the sensor temperature constant at 9 C; when the sensor temperature was allowed to follow gas temperature, then the signal output was inconclusive. Figure 4 examines pressure in the cases of combustion and misfire. During the compression stroke, the sensor current signal diminishes on account of the increase in pressure which decreases the diffusivity of oxygen (D ~ 1/P tot ). Combustion causes a further reduction in diffusivity on account of the attendant increase in pressure, as seen by the decay in the diffusion limited current. In this way, it is possible to deduce pressure from the output of an electrochemical sensor. Note that a result similar to a misfire could also be due to a failure of the head gasket, rings, or an improperly seated valve. A misfire is temporary, however, whereas these hardware defects persist. The dynamic environment of an engine has fluctuations in mole fraction of oxygen, volume, and total cylinder pressure. Figure 5 shows simulation results of these parameters over the entire cycle. The large number of degrees of freedom (Yo, P, T) complicate the direct measurement of oxygen content using a conventional, current limited amperometric oxygen sensor on account of the dependence of diffusivity on both temperature and pressure (D~T 3/ / P tot ). One can, however, determine the total cylinder pressure during the compression stroke prior to combustion using a single cell, amperometric, diffusion-limited sensor. During the compression stroke, oxygen content is constant and known. The crank angle position and thus volume is generally always known from the crank angle sensor. If one determines the temperature or holds the temperature constant, the signal can be calibrated to determine total cylinder pressure. To determine temperature, one can use a thermistor. The heater electrodes are thermistors, showing temperature dependent resistivity. Alternately, by mounting the sensor on a large thermal mass and using a heater, one can hold the temperature relatively constant. Figure 6 indicates that during the compression stroke, the signal decreases on account of the lower diffusivity of oxygen. When the low signal persists past top dead center, one has a confirmation of combustion. A high signal indicates a misfire or equipment failure (head gasket, rings, or improperly seated valve). p. 4

5 The model yielded the following findings: Sensor temperature must be high (9ºC) when using a porous electrolyte in order to obtain acceptable conductivity. Ion transport path must be micron scale for fast response. Area of electrodes cannot be too large or diffusion will not be the rate limiting step. Temperature must be known or deduced to determine pressure. In brief, during the compression stroke prior to pilot fuel injection, one can determine variables such that the only remaining parameter is pressure, which can be deduced by calibrating a galvanometric signal by engine mapping techniques to determine an heuristic algorithm for pressure. B. Laboratory Results Figure 7 and Figure 8 below show that the current response signal decreases as pressure increases. This is the expected result of pressure s effect on diffusivity. With these calibration graphs, one can determine the pressure for any particular bias potential as long as temperature is known or deduced via thermistor. The planar sensor exhibits a larger current response at every level of bias and pressure, probably due to larger surface area of the electrodes. We have demonstrated the use of conventional oxygen sensors to detect pressure. A single cell, analog oxygen sensor can be calibrated to determine pressure during the compression stroke prior to combustion in lean direct injection engines. Future Work Future work will involve designing electrochemical sensors that can be inserted into diesel engine cylinders using the existing ports. We intend to use a combination of ceramic processing and soft lithography methods to fabricate a planar sensor with an electrolytic path small enough to measure pressure variation in real time. We will analyze this sensor under controlled conditions and demonstrate its operation inside our experimental CFR engine. Conclusions In summary, we are developing a revolutionary solid state sensor technology for in cylinder gas analysis. Our research aims to decrease fuel consumption and emissions of lean direct injection engines by locating a sensor inside each cylinder in order to monitor pressure. Our simulation work indicates that an oxygen sensor reveals pressure on account of changes in diffusivity as long as temperature is established. Our experimental work with conventional oxygen sensors confirms their capability to detect pressure as predicted by our model. p. 5

6 Pressure (atm) Ideal Diesel Cycle P-V Diagram Volume (cm^3) Current (A) Limiting Current for Cycle when Tsensor = Constant.3 Intake Compression Power Exhaust..1 Misfire Combustion Crank Angle (deg) Pressure (atm) Figure 3. Simulated Diesel cycle Pressure-Volume diagram Pressure vs. Crank Angle Intake Compression Power Exhaust Misfire Combustion Crank Angle deg) Figure 4. Simulated pressure wave in diesel cylinder. Normalized Parameters Variation of Properties over 4 Strokes Intake Compression Power Exhaust 3 1 Total Pressure Crank Angle (deg) Mole Fraction O Piston Volume Figure 5. Trends in physical parameters over stroke. Current (ma) Figure 6. Simulation of oxygen sensor as pressure sensor on compression stroke and misfire detector on power stroke. Pressure Dependence of Biased Thimble Oxygen Sensor Output P (atm) Bias Voltage (mv) Figure 7. Output of Bosch 1376 thimble sensor is dependent on P. Current (ma) Pressure Dependence of Biased Planar Oxygen Sensor P (atm) Bias Voltage (mv) Figure 8. Output of Bosch planar sensor is dependent on P.References 1 Riegel, J., H. Neumann, H.-M. Wiedenmann, Exhaust gas sensors for automotive emission control, Solid State Ionics, ,, p. 785, F.T. Ciacchi, K.M. Crane, S.P.S. Badwal, Evaluation of commercial zirconia powders for solid oxide fuel cells, Solid State Ionics 73, 1994, p. 54, Keiji Yamahara, Tal Z. Sholklapper, Craig P. Jacobson, Steven J. Visco, Lutgard C. De Jonghe, Ionic conductivity of stabilized zirconia networks in composite SOFC electrodes, Solid State Ionics, 176, 5, p. 1363, p. 6