Research of combustion in older generation spark-ignition engines in the condition of use leaded and unleaded petrol

Abstract

This paper analyzes the potential problems in the exploitation of the older generation of spark-ignition engines with higher octane number of petrol (unleaded petrol BMB 95) than required (leaded petrol MB 86). Within the experimental tests on two different engines by applying piezoelectric pressure sensors integrated with the engine spark plugs, acceleration sensors and special electronic block connected with distributor, show that the cumulative first and second theoretical phase of combustion when petrol of higher octane number (BMB 95) is used lasts slightly longer than when the low-octane petrol MB 86 is used. For new petrol (BMB 95) higher optimal angles of pre-ignition have been determined by which better performances of the engine are achieved without a danger of the combustion with detonation (also called knocking).when unleaded petrol is used. Simultaneously, 1/3 to 2/3 of the total amount accumulates at the head of the piston [1]. Deposits in the combustion chamber are not only formed from petrol, but also from the used engine oil.Lead oxides in the deposits, as a kind of ceramics and heat insulators, reduce the effect of the convective heat transfer from the mixture to the chamber walls, reducing the level of the thermal load of the engine [1]. In this way, the working process also approaches the adiabatic process, which theoretically implies the better petrol economy of the engine. The part of the deposits accumulated on the valve seat and on the valve head has the role of a shock absorber, too, which is particularly important when the exhaust valves and their seats exposed to the high temperature of exhaust gases are concerned.Deposits formed by the use of unleaded petrol types do not have any of the above mentioned positive features of deposits originating from petrol with the addition of lead alkylates. Due to their chemical composition, deposits provide high-values of convective heat exchange between the mixture and the elements which form the combustion chamber, and their deep black color considerably increases the heat transfer by means of radiation [1]. This means that, in comparison with the deposits in the use of leaded petrol, we have higher temperatures of the elements which surround the engine combustion chamber and the higher temperatures of the cooling medium. These deposits do not have significant lubricating properties, nor the effect of the shock absorber when the valve hits its seat. The damage of the valve seat of engines has shown not to appear in the normal everyday use of the vehicle. The case of the damage of the valve seat which lead to the engine failure was noticed only on the laboratory acceleration tests, when the engine worked under the constantly high operation load [2]. In such conditions, it has been shown that the valve was not considerably damaged. Also, the previous accumulation of lead deposits and an increased amount of burnt oil deposits on the valves and the valve seats in older engines, prevent the occurrence of micro-welds in the contact metal to metal.On t...