Designing a Temperature-Based Heating System for an Automotive Engine Preheater

Overview

Winter can be very cold in Nordic countries; it is fairly normal to have temperatures below -20°C. Not everyone has a warm garage for their vehicles, so many people use an engine preheater, like the one shown in Figure 1.

Figure 1. An engine preheater. Image courtesy of DEFA.

The engine preheater is mounted into the vehicle (Figure 2) and typically heats the water inside the engine.

Figure 2. An engine preheater installed in a vehicle. Image courtesy of DEFA.

Starting a vehicle in a cold environment will put an engine under heavy stress. Starting an engine at -30°C puts the same wear on the engine as driving 600km (373 miles). Gasoline consumption during a cold start is much higher than the start of a preheated engine. Preheating the engine results in energy and cost savings, and creates less pollution.

There are two types of preheaters: one uses gasoline as energy and the other uses electricity. This article describes how to design a temperature-based heating system for an electrical engine preheater. Regardless of whether one uses an engine block heater, hose heater, oil heater, or contact heater—this temperature-based heating system described in this article will function correctly.

Recommended Preheating Time

Figure 3 shows preheating time as a function of ambient temperature. Preheating the engine is particularly relevant when the temperature is 5°C or lower. More than three hours of heating time is not recommended, as by that time, the engine is usually preheated up to its maximum temperature. If a user continues heating after three hours, valuable energy is wasted.

Figure 3. Engine preheating time versus ambient temperature.

Engine preheaters can be turned on manually or automatically with a timer. A user must manually turn on the system well in advance of the preheated start, because if the ambient temperature is very low, the recommended preheating time can be up to 180 minutes. This is not practical if the user needs to leave early in the morning. Alternatively, a user can program a timer to connect electricity to an engine preheater at a preordained time. This would require constantly checking the weather forecast to decide the optimum preordained time. Some car owners just set their timer for 120-minute preheating time, regardless of the ambient temperature. But this practice results in wasted energy.

This application note discusses how to design a convenient and efficient system that knows the desired engine starting time and ambient temperature. This system then uses that information to start the engine preheater at precisely the correct time to heat the car before starting, resulting in maximum energy and cost savings.

Temperature-Based Heating-System Description

A temperature-based heating system needs to measure the ambient temperature. The system should also know both the current time and desired engine starting time. The user interface is, of course, a must to tell the system the desired starting time, which may not always be consistent. The system should also connect and disconnect electricity to and from engine preheater. Because of possible power cuts, time information would need to be backed up with a battery. When all is put together, these components result in a system block diagram, like the one shown in Figure 4.

Figure 4. A block diagram for a temperature-based heating-system.

System Schematics

The system described in this article was compiled using two PCBs: One for the power supply and relays, and the second for the processor (digital) environment. A power supply and relay board schematic is shown in Figure 5.

Figure 5. A power supply and relay board schematic.

Power supply and relay board connects mains supply to relays and provides isolated, rectified, and preregulated 12V supply voltage to the processor board. The processor environment schematic is shown in Figure 6.

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Figure 6. The processor environment schematic.

DC-DC conversion from 12V to 5V is done using a linear low-dropout voltage regulator like the MAX16910, because it has built-in thermal and short-circuit protection. The system microcontroller is a Microchip PIC18F4520 running at an 8MHz clock frequency. Microcontroller clocking is completed using a very small (SC-70) silicon oscillator like theMAX7375, which has an excellent temperature coefficient and extremely low jitter.

Ambient temperature is measured with an analog-output temperature sensor. The MAX6605 has low output impedance(240Ω), so it can be connected directly to the microcontroller analog-to-digital converter (ADC) without an external bufferamplifier.

The system needs to know the exact time, so a real-time clock (RTC) is used. The Maxim DS3232 RTC has an internal oscillator and EEPROM. In this system, EEPROM is used to store timing parameters, so system information can be fully restored if a power cut event happens. The RTC is accessed using an I²C interface. Backup battery ensures that the system keeps accurate time information, even during a power outage. Additionally, since relays in this system require a 12V coil voltage, relay driving is done with NPN transistors Q2 and Q3.

Buttons (ON/OFF, SET, PLUS, and MINUS) are connected to the microcontroller with connector J2. The system display is a 4x20-character LCD and is connected to the microcontroller with connector J5. An LCD backlight is driven on/off bytransistor Q1. Connector J4 programs and debugs the microcontroller. Transistor Q4 is only gives 5V output voltage, if needed. The 5V output voltage can be used, for example, to drive an external LED to indicate if the system has connected electricity to the engine preheater.

System PCBs

System PCBs are shown in Figures 7 to 9.

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Figure 7. Component placements. The power supply and relay board is on the left side, and the processor board is on the right.

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Figure 8. Top-side layers.

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Figure 9. Bottom-side layers.

PCBs are mounted inside waterproof industrial enclosure with IP65-IP67 protection class. A fully functional and assembled system is shown in Figures 10 and 11.

Figure 10. A fully assembled and functioning system in the real world.

Figure 11. A functional system with an open front cover.

User Interface

The user interface consists of a 4x20-character LCD and four buttons: ON/OFF, SET, PLUS, and MINUS. The ON/OFF button manually connects or disconnects electricity to/from the engine preheater. The SET key accesses the settings for programming the desired engine start time, while PLUS and MINUS increase/decrease values and parameters. LCD row 1 displays date, time, and ambient temperature. Row 2 displays whether the starting time has been set. Row 3 displays the preordained starting time. Row 4 displays if heating is on or off. The user interface is shown in Figure 12.

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Figure 12. The user interface.

Part Lists

The power board and processor board part lists are shown in Table 1 and Table 2, respectively.

The LCD is 4x20 characters. Any HD44780-compatible LCD display will work. Check the LCD backlight polarity before connecting it to the PCB, because manufacturers sometimes use different pins. The backup battery is 2 × 1.5V AA batteries in series, and the buttons are 4 × normally open type.

Summary

This article explains how to design hardware for a heating system that connects electricity to an engine preheater, based on ambient temperature and a preordained engine starting time. This system can also be used for other purposes, such as turning on a storage heater if the ambient temperature falls below a preordained value.