Advanced electronic component reliability is allowing for better configuration and higher performance of marine horsepower engines. Temperature monitoring at critical engine locations provides important information on running behavior, safety margins and the need for maintenance on critical components like cylinder liners, exhaust valve seats and bearings. Even mobile parts of the gear train can be monitored for temperature thanks to wireless technology. Excessive temperature values and gradients could be an early indicator of imminent component failure.
So, it follows that temperature sensors are usually the most costly and numerous sensor type, often found at several locations and circuits on the same engine, measuring a range of parameters including oils, fuels, water, gases and exhaust. The three established sensor technologies of thermocouples, resistance temperature detectors (RTD) and thermistors are often combined on the same engine, each with their own advantages and drawbacks.
Temperature
Thermocouples have a high temperature capability ideal for measuring exhaust gases, cylinder head outlet, and turbocharger temperatures. They can also be used on fluids and engine components such as bearings, liners or exhaust valve seats when short response time is of interest and unobtrusive design is required.
Thermocouples give the shortest response times due to a small thermal inertia. They also offer the smallest size intrusion and are very robust against vibrations and thermal shocks. The drawback however is with accuracy as the relationship between temperature and voltage is not linear and depends on material quality and purity. This can be kept under control thanks to standards like CEI 60584-1.
Another disadvantage is related to installation cost. Thermocouples are required to compensate for the ‘cold junction’ by using a compensation cable, made of cheaper materials than the thermocouples themselves, so that no parasitic Peltier effect occurs at any cold junction connections along the wires. Compensation is sometimes managed by using additional temperature measurement in the junction box. Other drawbacks are found in the electronics as the lower voltage needs to be amplified to a few volts to allow reading on standard human-machine interfaces (HMI).
RTDs
RTDs mostly employ platinum elements and are used either at PT100 or PT1000, but seldom as PT200. In this designation, the number following letters ‘PT’ indicates the ohms value at 0°C. These are electronic resistances that show resistance value depending on their temperature according to a linear curve; higher temperature goes with higher resistance.
They are also low cost and accurate, typically adhering to CEI-6075 standard. Also, extension cables for connection to the HMI use low cost copper lines. PT100 continues to be popular for fluid temperature measurement, but needs to be connected by three or four wires to get the required accuracy. However, PT1000 sensors are becoming increasingly prominent as connection line resistance is comparatively negligible and does not affect accuracy.
Sturdy sensor housing and installation for RTDs are highly important as the platinum element can be sensitive to vibrations. RTD sensors are mainly used for fluids, compressed air and bearings as high temperature capability in the region of 600°C is limited and response times can slow due to thermal inertia.
Whilst their lead-wire resistance does not affect sensor accuracy, thermistors are sensitive to temperature, typically having much higher nominal resistance values than RTDs. These low cost ‘ready for use’ electronics are often used in automotive applications. However, not all automotive sensors can be used on industrial engines, which use much larger pipes and tubing than automotive engines, and which requires much higher lifetimes.
For example, marine engines with 4,000 hours of operation represents only around nine months of running. However, 4,000 hours of running for a car will bring them close to the end of their life cycle. With time between major overhauls for industrial engines varying from 30,000 to 50,000 hours, it is a challenge for fragile sensors to last this long with heavy day-to-day use.
Develop Solutions
Instrumentation manufacturers are working with engine OEMs to develop solutions to meet the reliability and lifetime challenges for industrial engine sensors, while trying to minimize engine delivery cost.
The evolution of engine technology makes the importance of reliability and long lifetime for exhaust gas temperature sensors increasingly critical. As reliable sensors are now mandatory for today’s common rail and electronic gas engines, any sensor failure, even for a few milliseconds, is unacceptable - failure of exhaust gas temperature sensors can lead to the immediate shut down of the engine.
Exhaust gas temperature sensors are now mandatory for smaller engines with electronic injection, offering considerable business opportunities to a few instrumentation makers that can produce sensors with excellent reliability. This situation will not last forever. New alternatives have been developed since 2010 to overcome the weaknesses of low cost PT1000 sensors by incorporating sought after features such as fast response time, long lifetime, vibration resistance, high temperature reliability, small diameter and low wiring and integration costs.
These solutions use miniaturized electronic convertors attached to each sensor, transforming analogue signals to digital ‘CAN’ protocol, conforming to ISO 11898 and SAE J1939 standards. Also, a simpler measurement harness, reduced to 4 wires instead of dozens, can be directly connected to the HMI using only one standardized port dedicated to digital input, relegating the need for an HMI acquisition box.
Builders of advanced digital protocol engines with electronically injected fuels can embrace the latest digital technology and will benefit from cost advantages and the reliability gained through rigorous testing and qualification programs. Engine builders for commercial applications are recognizing the flexible advantage of digital technology that allows fast implementation of additional sensors, without any physical change in input/output ports of the Engine Control Unit (ECU), allowing additional reserves of sensors imbedded in the ECU. The complexity of harness design is also reduced as any additional sensors simply require an additional harness connector, without an increase in harness size or additional wiring.
About the author
Patrice Flot is the chief technical officer at CMR Group. Prior to this, he held various positions at the company, including operations manager, while also overseeing the R&D, production, design, and technical departments. Before CMR, he was the technical manager at Wärtsilä France in Mulhouse. He was educated at Ecole Nationale Supérieure des Pétroles et Moteurs and the Ecole Polytechnique in Paris. More info is available at www.cmr-group.com.