PART 3 - Results
OIL STRATEGIES BENEFITS OVER DIFFERENT DRIVING CYCLES USING NUMERICAL SIMULATION
1st AREA: THERMAL MANAGEMENT
Abstract
95g/km is the allowed quantity of grams CO2 per kilometer normalized to NEDC to be set in 2020. In addition, NEDC will be replaced by more severe driving cycles and will be united worldwide. To respond to those criteria, automotive industries are working on every possible field. Thermal management proved its efficiency in reducing fuel consumption. Cold start is a key reason of overconsumption, as the engine highest efficiency is at its optimal temperature. At cold start, the engine’s oil is at its lowest temperature, thus its higher viscosity level. A high viscosity oil generates more friction; one of the most important heat losses in the engine.
As a consequence, a study of hot oil storage is undertaken. Numerical simulations were done. The model consists of a 4-cylinder turbocharged Diesel engine using a storage volume of 1 liter of hot oil. Different ambient temperatures were taken into consideration as well as different driving cycles. Further, different configurations of the thermal strategy (multifunction oil sump) were proposed and evaluated. Lubricant temperature and viscosity profiles are studied as well as fuel consumption gain for different configurations, driving cycles and ambient temperatures.
Engine modelling
A turbocharged 4-cylinder Diesel engine is used in this study. The four cylinders are in line position with a compression ratio of 15:3 and four valves per cylinder. The engine was equipped with common rail fuel injection system, high EGR loop, turbocharger with a VGT and charge air cooler. Different engine tests were held in order to calibrate the engine model. Cold starts were taken into consideration to calibrate the thermal part of the engine. The engine model can be separated into two different parts. The first one is defined as a high-frequency model simulating the air flow through the intake line, through the engine where it’s mixed with the right amount of fuel to inflame the mixture, leaving then the cylinder, and passing by the turbocharger and the exhaust line before being evacuated to the ambient air. Heat losses to the combustion wall were taken into consideration. The second part covers the hydraulic and thermal part of the engine where the coolant and lubricant circuits are modeled. A traditional thermostat controls the coolant circuit where it opens the radiator branch at the optimal temperature of 83°C. Three heat exchangers are modeled, covering the heat transfer between the coolant and the EGR loop, the ambient air, and the oil at the oil cooler, where the two hydraulic systems exchange heat either to cool down the oil or heat it up in the early stages of the driving cycles. The oil circuit covers the oil pump where it sucks oil from the sump to the oil/coolant exchanger then to the engine where it receives the heat either from the friction power or through heated parted of the engine as pistons. The oil circuit takes into consideration the pressure losses in the system (filter, engine, ducts …) where it is modeled to have the right flow rate in the system. The two models functions in a loop, the high-frequency provides the heat fluxes crucial for the function of the coolant and the lubricant circuits, at its turn the latter returns different thermal masses’ temperature as well as the lubricant real temperature used to the correction of the friction model of the GT-Suite.
Fuel consumption saving on driving cycles
The homologated driving cycle imposes a cold start. Within a cold start, the friction power losses are important as a consequence of higher oil viscosity due to lower oil temperature. The oil viscosity decreases exponentially with its temperature; at first an important drop of the viscosity is obtained with increasing slightly the temperature to a point where it tends to be almost constant at some temperature level. Taking in charge the start of the cycle, where the oil temperature is the lowest and the friction losses are the highest, is one remedy to lower fuel consumption. Oil’s temperature at the end of the driving cycles reaches a minimum of 95°C. Storing hot oil from one driving cycle, in order to use it at the cold start of the next one is the thermal strategy proposed in this study.
This thermal management strategy is tested over two ambient temperatures: 20°C presenting a normal warm day and -7°C, a severe winter. The hot oil storage was tested over different driving cycles in order to assess the gain for different living style. NEDC (New European Driving Cycle) is the first driving cycle to be tested as it’s the homologated driving cycle used nowadays to assess the different emissions and fuel consumption. The second driving cycle is WLTC, globally harmonized and will be replacing the NEDC as the homologation driving cycle. Artemis highway (AH) was the third on the list. It gives a slight change in the representation of the engine, where it’s a more severe driving cycle, with an engine at high load point at the transient phase. Another cycle was chosen; Artemis Urban (AU), reproducing urban European average real world driving cycle. Finally an in-house developed cycle (HDC), as depicted below, was proposed as a representative of a suburban driver taking the car to work every morning. It is a short driving cycle of 254s for 2 km, reaching 90 km/h only after 50s and then decelerating at some point due to the arrival at the work place.
In-House Developed Cycle (HDC)
Three other configurations were studied. Configuration A consists in integrating the storage volume in the oil sump. In addition, this one will be isolating one liter of oil already existing in the system. Configuration B is based on the previous configuration, where the storage will be done in the oil sump, with addition neither of mass nor of oil volume. The difference is in the control, where one criterion will be added. As the friction is a result of two parts in contact sliding against each other, and as the moving parts' speed of the engine is proportional to its rotational speed; the valve of the storage tank will be controlled by the rotational engine speed, in addition to the oil’s temperature. This configuration takes into consideration the safety of the moving parts, as with higher rotational speed, higher is the flow rate of the oil pumped into the system. In order not to isolate 1L of oil during the peak point of flow rate and depending on the different driving cycles used in this study, 2500 rpm is the level where the valve controlling the storage part of the sump opens up to back up the flow of the oil into the system.
Configuration C is the simplest among them all, called no valve configuration. The oil sump is equipped with an isolated part where the hot oil should be stored through a soak period, and during the function of the engine, oil flow rate from the isolated part as well as from the other part will be mixed upstream the pump.
The fuel consumption result of the different configurations for the different cycles and different ambient temperatures is shown below. The main outcomes are the importance of the oil strategy proposed on the coldest winter days and the third configuration where it proved that isolating a part of the oil sump will lead to some improvements in fuel consumption.
Configuration C is the simplest among them all, called no valve configuration. The oil sump is equipped with an isolated part where the hot oil should be stored through a soak period, and during the function of the engine, oil flow rate from the isolated part as well as from the other part will be mixed upstream the pump.
The fuel consumption result of the different configurations for the different cycles and different ambient temperatures is shown below. The main outcomes are the importance of the oil strategy proposed on the coldest winter days and the third configuration where it proved that isolating a part of the oil sump will lead to some improvements in fuel consumption.