Solid-State 1H NMR Spin-Lattice Relaxation in Combustion Chamber Deposits from a Gasoline Direct Injection Engine

Combustion chamber deposits from a gasoline direct injection stratified charge SI engine were analyzed using solid-state ¹H inverse recovery NMR spectroscopy, before and after thermal desorption (TD). The engine was run with a typical European base fuel, containing 9.8% MTBE.

Before TD, solid-state ¹H NMR showed two broad peaks representing aliphatics and aromatics. The results showed that T₁ (the relaxation time) of the aliphatics was significantly longer than T₁ of the aromatics in the deposits. Deposits taken from the piston bowl, with the lowest volatility content, showed the shortest T₁. In comparison, piston squish and piston bowl deposits, with higher volatility content showed a longer T₁. After TD, T₁ of the aliphatics was shorter than the corresponding T₁ of the aromatics. A decreasing T₁ of aliphatics may be used for following a transition from a liquid (engine oil), via a semi-solid deposit to a solid carbon backbone. This may give a rough measure of the engine oil content of the deposits.