The fire point of the diesel is greater than petrol and compression ratio of diesel engine is greater than petrol engine. If we use petrol on the diesel engine, since diesel engine has greater compression ratio the air heated during the compression has temperature which is enough to burn the diesel, but here we use petrol, the petrol injected at the end of compression would burn immediately before the power stroke gets started unlike diesel to burn completely during power stroke. This due to excessive burning temperature of air for petrol, but for diesel it will be normal burning temperature. Hence piston can start to move to BDC before reaching the TDC during the compression stroke, this would reverse the engine and may cause engine vibration and noise. This is the reason for Why petrol cannot be used in diesel engine?.