B. Tipe Induksi
4.2.4 Spark plug (busi)
Spark plug, seperti tampak pada gambar 4-20, dipasang diruang pembakaran masing-masing cylinder head dan membakar campuran udara bahan bakar di dalam cylinder dengan cara membangkitkan bunga api diantara central electrode dan ground electrode menggunakan tegangan tinggi yang dihasilkan dari coil pengapian kedua.
Gambar 4-20 Posisi pemasangan spark plug (1) Struktur spark plug
Spark plug, seperti tampak pada gambar 4-21, terdiri dari tiga komponen utama yaitu electrode, insulator dan shell.
Gambar 4-21 Structure of the spark plug Electrode
Electrode terdiri dari central electrode dan ground electrode. Dikarenakan tegangan tinggi diinduksikan dari ignition coil kirim ke central electrodes melalui central shaft, maka pengapiannya akan dihasilkan dari celah ground electrode. Celah antara central electrode dan ground electrode adalah 0.7~1.1mm. Bahan untuk membuat electrodes harus yang kuat, tahan panas dan tahan karat sehingga materialnya terbuat dari nickel alloy atau platinum alloy. Dalam hal tertentu, karena pertimbangan radiasi panas, central electrode bisa terdiri dari tembaga. Diameter central electrode umumnya adalah 2.5mm. sekarang ini, untuk mencegah terjadainya pengapian rendah dan untuk meningkatkan performa pengapian, beberapa central electrodes mempunyai tingkat ketipisan pada central diameter kurang dari 1mm atau pada ground electrode-nya berbentuk alur U.
Insulator
Insulator berfungsi untuk menghindari terjadinya kebocoran tegangan karena central electrode atau core, sehingga alat ini mempunyai peranan yang penting dalam menentukan performa pengapian. Karena itulah, insulator mempunyai daya isolasi terhadap listrik yang cukup baik, tahanan panas, kuat dan stabil. Insulator ini terbuat dari keramic yang mempunyai daya sekat
yang baik serta mempunyai penyangga untuk mencegah terjadinya flashover dari arus tegangan tinggi.
583,731 Shell
Shell adalah komponen metal yang mengelilingi insulator dan sekurup untuk bisa dipasang pada cylinder head. Ground electrode disolder pada bagian ujung sekrup. Sesuai dengan diameter sekrupnya, terdapat 4 macam sektrup 10mm, 12mm, 14mm dan 18mm. Panjang (jangkauan) sekrup ditentukan oleh diameternya. Untuk panjang sekrup 14mm, terdapat 3 jenis sektrup 9.5mm, 12.7mm dan 19mm. Celah antara insulator dan inti kawat atau shell didempul oleh sealant khusus yaitu glass seal (setelah glass dan copper powder dimasukkan ke dalam joint part pada central electrode dan core kemudian dicairkan untuk dipasang insulator dan metal) atau dilarutka bersama dengan spark.
(2) Persyaratan yang harus dimiliki busi
Spark plug mempunyai sktruktur yang sederhana dimana dua electrodes dari sirkuit pengapiannya saling menghadap satu sama lain untuk membuat loncatan bunga api. Namun demikian kondisi lingkungan kerja sekitarnya sangat keras sehingga performanya harus bisa memenuhi kondisi sebagai berikut :
Harus tahan terhadap panas
Spark plug dibakar diruang pembakaran yang mempunyai temperatur sekitar 2,000 derajat celcius dan didinginkan secara tiba-tiba oleh langkah hisap melalui gas yang diinjeksikan. Karena itulah busi harus bisa tahan terhadap perubahan temperatur yang drastis.
Konstruksinya harus kuat
Spark plug dipengaruhi oleh adanya getaran yang cukup kuat karena adanya perubahan tekanan pada vacuum pressure di langkah hisap dan sekitar 35~45kgf/cm dilangkah buang. Maka dari itu busi harus mempunyai daya tahan yang cukup kuat.
It should have good corrosion resistance
The electrodes of the spark plug are exposed in the combustion gas so they can be chemically attacked by the carbon and so on. Therefore, it is needed to endure the corrosion. Generally, they are made of Ni-Cr alloy.
It should have good airtight-ness
The spark plug should have the airtight-ness enough to endure the pressure applied from the compression and expansion strokes. Especially, there is no gas leakage at the high temperature.
It should keep the self-cleaning temperature
If the temperature of the electrode is extremely increased, then it will be a reason of the advancing spark, otherwise if it is too low then carbon slug will be stacked and current leakage will be occurred. These are the main reason of misfire. Therefore, during the engine operation, the temperature of the electrode should be maintained in 500~600 derajat celcius.
It should have good electric insulation performance.
The spark plug should endure against the high voltage of 15,000~20,000V during engine operation and maintain good insulation property under sudden change of temperature. It generally is made of alumina (AlO3) or other magnetic insulating materials.
The spark should have strong energy
As the end part of the electrode is sharp as possible, the spark will be easily made. If the end part of the electrode is too sharp, then it has short lift time. Therefore, it should have proper shape to make the spark smoothly.
It should have good ignition performance
Even the electrode makes a spark, if the energy is not sufficient, then the firing is not accomplished. Therefore, the shape of electrode should be considered to make sufficient energy in the lean mixture of fuel gas.
It should have good heat conduction
If the heat from the combusted gas is not radiated, the electrode could have short life time from the melting or corrosion. Therefore, it has heat conduction enough to maintain the temperature being under 950 derajat celcius. Especially, at the high temperature, it has high heat conduction efficiency.
(3) The self cleaning temperature and the heat value of the spark plug
During engine operation, as the spark plug is exposed in the high temperature by the combustion of the fuel mixture, the electrode should maintain proper temperature. If the operating temperature of the electrode of the spark plug is lower than 400•, then the carbon made from the combustion will attach to the electrode so that the insulation property will be degraded and the spark will be weakened, finally the misfiring is occurred. If its temperature is over 800~950•, then the firing time will be advanced so that the engine output will be lowered. Therefore, the most proper temperature of the electrode is 500~600•. This temperature is called the self-cleaning temperature of spark plug.
The self-cleaning temperature is decided by the applied heat capacity and the radiated heat capacity. The applied heat capacity is varied by the type of engine and driving status, and the radiated heat capacity is varied by the structure of spark plug. As the spark plug has different radiated heat capacity property contrast in its structure, it should be carefully selected for the engine. The numerical expression of the radiation heat capacity is the heat value. The heat value is decided by the length from the part just below of the insulator to the lower seal. The used heat value of spark plug is very important and varied by the style of combustion chamber of engine, the position of intake-exhaust valve, the compression ratio, and the rotation speed. With the same material, when the area exposed to the combustion gas is large and the radiation path (length of the all part of insulator) of the heat is long, the radiation property is inferior and it is easy for the temperature to increase. This type is called hot type.
The characteristic of the hot type spark plug is large resistance against the damage but low resistance for the advancing ignition. Therefore, it is proper to the low speed and low load engine. The cold type has high heat radiation property and low temperature increasing. The characteristic of the cold type is that this type has high resistance against the advancing ignition but it has low damage resistance. Therefore, it is proper to the high speed and high load engine.
Sekaran ini sudah banyak macam busi yang bisa mempertahankan temperatur suhunya dalam berbagai macam kondisi berkendara. Umumnya batas ketahanan panasnya tertulis pada businya untuk memberikan informasi mengani tipe dan ukurannya. Angka penunjuk panasnya akan bermacam tergantung dari pabrik pembuatnya. Angka yang besar berarti jenis busi dingin, sedangkan angka kecil berarti jenis busi panas.
Gambar 4-23 Radiasi busi
4.3 DLI (Distributor less Ignition)