Raw Engine Exhaust Mass Flow Meter
Sierra CP has successfully manufactured and sold AirTrak intake air mass flow meters since the early 1990's. With deep roots in flow measurement through our Flow Division, we have now entered the market with an innovative raw engine exhaust mass flowmeter, named ExhaustTrak, to address this tough application. ExhaustTrak is directed towards satisfying customer requirements for a rugged, accurate and fast-response time exhaust flowmeter. The instrument is specifically designed for transient cycle flow rate measurement.
In addition to raw exhaust (pre or post-aftertreatment), the instrument is ideal for intake air mass flow, diluted exhaust, or crankcase combustion fumes. Taking clues from our use of porous media (sintered stainless steel) in our patented partial flow dilution tunnel utilized by the BG3, we use circumferential porous media as pressure interface material to elevate subsonic venturi flow meter technology to a whole new level. The design provides fully averaged radial pressure sampling at the throat and inlet locations. Flow inaccuracies due to velocity profile skew are minimized. As a result, straight length requirements for installing the ExhaustTrak are relaxed, as it is nearly immune to bias effects created by bend-induced swirl.
WHAT IS IT?
- In-situ direct exhaust mass flow measurement
- Provides accurate, real-time exhaust mass results
- No chemical balance analysis needed
- Produces proportional analog signal for PM sampling devices (BG3) or other
- Produces output in engineering units to host
- Fuel H/C input, direct O2 measurement capability allows for real-time exhaust molecular weight compensation
WHY MEASURE DIRECT EXHAUST MASS FLOW?
Direct, in-situ measurement of exhaust mass flow for raw emissions sampling is difficult to execute. Given the allowance for chemical balance-derived exhaust mass flow for raw emissions sampling in 40CFR Part 1065, direct measurement was initially considered unnecessarily problematic. However a recurring observed bias between raw and dilute gaseous emissions results on transient test cycles has prompted reconsideration of this position.
One drawback to the use of chemical balance methods is that instantaneous exhaust mass flow rate is not available until after the emissions test has been completed. Chemical balance post-test data processing may be complicated due in part to temporal delays in gaseous emissions measurement system response, created by sample line length and/or individual analyzer response times, relative to an engine operating event such as an acceleration or load acceptance. This approach may be further complicated by required changes in engine exhaust mass flow rate due, for example, to an engine power rating change. Further, modifications of the exhaust system consist which may affect intake or exhaust stack volumes, or a change in after treatment device status, may be particularly problematic. Such changes may create skew, or temporal redistribution, for example, in the relationship between measured combustion air mass flow rates and the actual instantaneous exhaust mass flow rate at the sample zone. Each of the aforementioned elements may contribute to uncertainty in measurement of both exhaust gas mass flow rate and raw engine emission results during engine development and certification.
Due in part to the elements discussed above, it is believed by Sierra CP that enabling fast and accurate exhaust mass flow rate determination through direct measurement is a necessary step to enable and expand acceptance of raw emissions measurement methods. It is our position that the most robust method for determining real time exhaust mass flow rate is to use direct exhaust mass flow rate measurement with a minimum 10Hz update rate capability. ExhaustTrak is our answer to this challenge.
BENEFITS & FEATURES
- Sub-Sonic Venturi Technology (Patent Pending)
- Eliminates need for chemical balance mass methods
- Eliminates instantaneous emissions error contributions due to indirect vs. direct exhaust mass flow determination
- Resolves temporal offset vs. intake mass sources through direct measurement at emissions sampling zone
- Will communicate with any test cell host
- Plug and play with BG3 provides best possible proportional sampling of particulate matter
- Wide range of flow rate capabilities available
- 1065-recognized exhaust mass measurement technology
- Fully averaged radial pressure sampling
- Porous media as pressure interface
- No plugging issues
- Reduced straight length requirements
- Rugged, accurate & fast measurement
Each ExhaustTrak requires precise application data to assure optimal sizing and performance. The following information will be required to properly fit the instrument:
- A list of engine sizes to be measured along with exhaust stack diameters
- Engine flow ranges
- Temperature ranges of exhaust at sample point
- Ambient temperature in test area
- Back pressure at the sample point, specifically: 1) How much back pressure is applied to the point of exhaust flow measurement by post measurement devices, if any? 2) How much back pressure is allowed to be added with the installation of the ExhaustTrak, is there a limit?
ExhaustTrak is a sub-sonic venturi, engineered for measurement of particle-laden exhaust streams applicable to raw or diluted exhaust, crankcase fumes or combustion air. The mass flow meter benefits from a circular arc throat approach and tapered recovery section and uses short radial porous stainless steel rings for pressure sampling integral to and concentric with the body, at standard venturi pressure measurement locations. The instrument communicates pressure to external averaging rings and pressure transducers. The radial porous rings are machined and processed to provide consistency in diameter versus the parent body material. Other than a slight change in surface finish at the pressure measurement zones, the porous sections are undetectable by sight or feel (no sharp edges). This feature provides radial pressure averaging at the high and low pressure measurement zones and straight length installation requirements relaxed. Due to boundary layer shear, PM build-up on the porous section surfaces is negligible. ExhaustTrak is equipped with a purge mechanism that can be either manually or automatically activated to address any build-up concerns.
- Accuracy <1.5% reading
- Pneumatic signal delay due to porous rings is less than 10 milliseconds
- Permanent pressure loss is less than 10% of delta-P
- Exhaust-Trak is custom-sized by exhaust pipe diameter and engine back pressure requirements.
- Typical peak back pressure is less than 1kPa
- Fuel H/C is user-definable in the Exhaust-Trak software
- Exhaust-Trak uses user-inputted fuel H/C and measured exhaust O2 readings to determine exhaust molecular weight (density) and provide >10Hz flow rate output (supports BG3 transient sampling, raw gaseous measurement)
- Exhaust-Trak pressure, temperature instrumentation is separately calibrated against reference standards
- Venturi body coefficient of discharge, (Cd) vs. Reynolds number (Re#) is determined using traceable flow standards, on air (flowing calibration)
- Exhaust-Trak instrumentation and a generic Cd-Re# curve-fit are active
- The calibration-based Cd vs. Re# results are curve-fitted and the calibration-generated coefficients replace the generic curve-fit in the software
- This method resolves small calibration errors from the individual transducer calibration, creating a Exhaust-Trak system calibration directly traceable to reference standards
- Easy to use computer interface software
- System is installed at desired measurement location
- User inputs fuel H/C into setup screen
- Establish serial communications with host, BG3 (if applicable)
- System is ready to measure