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Diesel Tuning Fundamentals: Air Fuel Ratio in Diesel Engines

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Air Fuel Ratio in Diesel Engines


00:00 - One of the most important aspects of diesel engine tuning is that we control the power and torque the engine produces based on the amount of fuel that we deliver.
00:09 With this in mind, the air fuel ratio is understandably an important aspect for us to monitor when it comes to tuning the fuel delivery.
00:17 For those of you coming from a gasoline tuning background, you may be used to running air fuel ratios in the range of perhaps 0.6 to 0.7 lambda at the very rich end of the scale, through to perhaps 1.1 through to 1.3 lambda at the lean end of the scale.
00:33 Within reason, the air fuel ratio is not a major factor in the engine's torque production and instead torque is predominantly controlled by veering the air flow into the engine.
00:44 In particular with a gasoline engine there is a lean air fuel ratio limit where we can no longer reliably ignite the air fuel charge.
00:53 Our aim therefore is simply to provide a gasoline engine with the correct amount of fuel to mix with the mass of air entering the engine at any point.
01:02 On the other hand as we've already discussed, diesel engines don't require the air flow to be throttled and we can achieve reliable combustion with air fuel ratios that are much leaner than stoichiometric.
01:14 On that note the stoichiometric air fuel ratio for diesel fuel is 14.5:1 however we will always be operating on the lean side of the stoichiometric air fuel ratio even under maximum power.
01:28 The general principle here is that we'll begin adding fuel until we've reached our desired engine torque level.
01:35 As we increase the fuel injected per cycle and richen the air fuel ratio though we will get to a point where the engine is producing large amounts of soot which can't be burnt prior to the exhaust stroke.
01:48 This results in excessive black smoke from the exhaust and this can become a limiting factor in terms of the fuel delivery and power production.
01:56 I'll note here that this is a very simplistic view of the process.
02:00 And we'll be diving deeper into this in coming modules.
02:04 As we richen the air fuel ratio we'll also be combusting more fuel and hence we'll see the combustion chamber temperature increase.
02:12 For our purposes in the aftermarket it's not possible to directly measure the combustion chamber but rather we measure it indirectly by way of exhaust gas temperature sensors.
02:23 Monitoring the exhaust gas temperature is therefore commonly used as a guide when adjusting the fuel delivery.
02:29 We'll discuss exhaust gas temperature more in an upcoming module so we won't cover it any further now.
02:36 As a general guideline we'll normally want to keep the air fuel ratio leaner than approximately lambda 1.1 to 1.2 or 15.9:1 through to 17.4:1 on the air fuel ratio scale under full power operation in order to control exhaust smoke and combustion temperature.
02:54 One of the unique aspects of tuning a turbocharged diesel engine is that we can use the turbocharger to provide an increase in air flow and in turn lean the air fuel ratio out for a given fuel volume.
03:07 This has the effect of also cooling our combustion temperature and providing additional oxygen which reduces exhaust smoke and in turn allows us to then provide more fuel and produce more power and torque.
03:21 In this way turbocharging is the key to significant increases in performance from a diesel engine.