How does lambda affect fuel consumption?

The engine fuel rate parameter ($5E) is supported only by rather new OBD2 compliant cars. The engine gives real-time data on how it’s consuming fuel with this parameter. But don’t worry even if your car doesn’t support it.

OBD Auto Doctor provides the calculated fuel consumption for almost every car. The software calculates the estimated fuel consumption based on several other parameters. The formula is a bit different depending on what parameters your car supports and what fuel it uses.

What is lambda?

In car diagnostics domain, air-fuel equivalence ratio is often regarded as lambda (λ). This is especially true in spoken language. Air-fuel equivalence ratio is the ratio of actual air-fuel ratio (AFR) to stoichiometry for a given mixture. In other words, lambda is the ratio between the amount of oxygen actually present in a combustion chamber versus the amount that should have been present to get perfect combustion.

  • In an ideal mixture, there’s exactly the amount of oxygen required to burn the amount of fuel present. Thus, lambda = 1.0.

  • In a lean mixture, there’s too much oxygen for the amount of fuel. Thus, lambda > 1.0.

  • In a rich mixture, there’s too little oxygen for the amount of fuel. Thus, lambda will be < 1.0.

In practice, the AFR devices measure the amount of residual oxygen or unburnt hydrocarbons in the exhaust gas. The engine monitors the lambda values in a feedback loop to adjust the air-fuel mixture.

OBD Auto Doctor uses the lambda values for the fuel consumption calculations. This is to get the most accurate estimation of fuel consumed.

Sources, and more information available at Air-fuel ratio on Wikipedia and Lambda as a Diagnostic Tool.

Lambda in practice

We did an experiment to find out how the lambda values affect the calculated fuel consumption in practice. In a data set A, we fetched the real lambda values from the car, and used those in the calculations. In a data set B, we assumed that the mixture was ideal all the time. In this case, the value of 1.0 was used as the lambda value.

We drove a sample track and measured both data sets during the same drive. Below is a graph containing the results. Click the image for larger view.


Fuel consumption with ideal and measured lambda
Fuel consumption chart from the csv data. Click for large image.


As you can see from the image, the ideal vs measured lambda doesn’t have too much impact in fuel consumption calculations. When driving at a constant speed, the difference is barely noticeable. During and after acceleration and deceleration, you can observe the change easier.

With this experiment, the average consumption with the ideal lambda was 5.69 [l/100km] while with the measured lambda it was 5.64 [l/100km]. So on average, the calculations are accurate even without the measured lambda value. However, with other engines and car models, the change can be different. This data was collected from 2.0 liter TFSI gasoline engine.





Torque, engine power and boost pressure added

A few days ago we launched the updated OBD Auto Doctor software. This time our main focus was in calculated performance parameters such as Engine Torque, Engine Horsepower and Turbo Boost Pressure.

You can now measure the performance of your car by monitoring these new sensors in real time. Read further to get insight into the details.


Torque, Engine Power and Boost Pressure
Engine performance sensors chart from the csv data, saved using Google Sheets


Engine Power or Horsepower

Engine power (or horsepower) can be estimated quite well using vehicle speed and its change over very short period of time. The total weight of the vehicle is also needed for valid results.

OBD Auto Doctor measures the engine power at the wheels. This is not the same as at the flywheel. Therefore, if the vehicle speed doesn’t change, the power will show zero. The horsepower will be greater than zero only when the vehicle is accelerating.

The software will also take power loss due to drag into consideration when calculating the power. An average drag coefficient and vehicle frontal area are used.

You can set the vehicle weight in the app settings. To get the most accurate weight, drive your car to a weightbridge. You could also estimate the total weight by using manufacturer defined weight added with the passenger weight, fuel weight plus any extra weight included in the vehile.

Torque

Torque is calculated from the engine power and RPM. With these two pieces of information, the torque can be estimated quite accurately. Torque is measured at the wheels just like the engine power.

Since torque is using engine power for the calculation, the vehicle weight is a factor in the calculation. Remember to set the correct weight!

Boost Pressure

Boost pressure value yields from the intake manifold absolute pressure. Barometric pressure is also taken into consideration. Now, the vehicle is required to support MAP (manifold absolute pressure) in order for the boost pressure to appear as supported. Barometric pressure is optional. The default pressure value at sea level is used unless the car provides support for the actual barometric pressure.

Negative pressure values represent vacuum. Positive values are boosted by a turbocharger or supercharger.

Other improvements

Among the new calculated performance parameters, we also added more.

The software has now support for all Mode $06 scaling IDs. The latest OBD2 standard version added 12 new identifiers. OBD Auto Doctor implements now all the new IDs in addition to the old ones.

If you are using the desktop software, download the latest version from the download page. The updated mobile apps can be found from the approriate app stores using your phone.





Diagnostic Trouble Codes Explained

Diagnostic Trouble Codes or OBD2 Trouble Codes are codes that the car’s OBD system uses to notify you about an issue. Each code corresponds to a fault detected in the car. When the vehicle detects an issue, it will activate the corresponding trouble code.

A vehicle stores the trouble code in it’s memory when it detects a component or system that’s not operating within acceptable limits. The code will help you to identify and fix the issue within the car.

Each trouble code consists of one letter and four digits, such as P1234. This blog post will teach you how to interpret the meaning of the codes.

Format of the OBD2 Trouble Codes

System or Category

The OBD2 Trouble Codes are categorised into four different systems.

  • Body (B-codes) category covers functions that are, generally, inside of the passenger compartment. These functions provide the driver with assistance, comfort, convenience, and safety.

  • Chassis (C-codes) category covers functions that are, generally, outside of the passenger compartment. These functions typically include mechanical systems such as brakes, steering and suspension.

  • Powertrain (P-codes) category covers functions that include engine, transmission and associated drivetrain accessories.

  • Network & Vehicle Integration (U-codes) category covers functions that are shared among computers and systems on the vehicle.

The first letter of the code will mark the system related to the trouble code.

Generic and manufacturer specific codes

The first digit in the code will tell you if the code is a generic or manufacturer specific code.

Codes starting with 0 as the first digit are generic or global codes. It means that they are adopted by all cars that follow the OBD2 standard. These codes are common enough across most manufacturers so that a common code and fault message could be assigned.

Codes starting with 1 as the first digit are manufacturer specific or enhanced codes. It means that these codes are unique to a specific car make or model. These fault codes will not be used generally by a majority of the manufacturers.

The first digit might be also 2 or 3. In this case the type depends on the system. B2xxx and C2xxx codes are manufacturer controlled while B3xxx and C3xxx codes are reserved at the moment. P2xxx codes are generic codes while P3xxx codes are manufacturer controlled. U2xxx codes are manufacturer controller as well as U3xxx codes.

Subsystem or functional area

Previously, the second digit defined the sub-system of the codes. However, the latest document defining the diagnostic trouble codes (J2012 revised in 2016-12) had some changes to this.

According to the document, as the DTC usage has increased with the introduction of new technology to vehicle systems, it was necessary to remove the grouping of DTCs into functional areas.

Fault description

The last two or nowadays three digits define the actual fault description. These numbers will tell the particular problem and each code is defined separately. There’s no formula to decode these codes automatically.

Luckily, OBD Auto Doctor software contains the fault description for over 18 000 diagnostic trouble codes.

Learn more

There’s no need to memorize the format of the codes because you can read the codes with the free version of the OBD Auto Doctor car diagnostic software.

If your car has the Check Engine Light on, it means that the vehicle has one or more confirmed OBDII trouble codes active. To learn

  • What to do when the malfunction indicator light illuminates, read this article.

  • How to read and clear the diagnostic trouble codes and the check engine light, read this article.





Thousands of new OBD2 Trouble Codes added!

OBD2 Trouble Codes or Diagnostic Trouble Codes are defined by SAE (Society of Automobile Engineers). SAE is the organization that specifies the OBD2 standards. These include the one that defines all the generic Diagnostic Trouble Codes (DTCs). Car manufacturers follow these definitions when they assign DTCs for detected errors.

New DTCs

J2012 is the document that defines the OBD2 Trouble Codes. The standard hasn’t been updated for a while but few months ago it got a new revision. The newest revision contained a bunch of new DTCs. These DTCs take new technology such as hybrid vehicles into better consideration, for example.

OBD Auto Doctor contains now all the possible generic OBD2 Trouble Codes that exists. We added total of 3899 new DTCs to the software.

The software contains total of 18193 Trouble Codes. The latest PC & Mac software as well as the Android and iOS apps have all these codes included.

The codes include:

  • Powertrain: 6631 codes
  • Network & Vehicle Integration: 1017 codes
  • Chassis: 363 codes
  • Body: 165 codes
  • Rest of the codes are manufacturer specific trouble codes

Multiple Freeze Frames

The latest desktop software version added support for multiple Freeze Frames too. Typically, only one freeze frame is saved. But with newer cars, it’s possible to store multiple freeze frames; one for each trouble code. The software will now read all available frames.

This is a great addition if the vehicle has more than one issue present. You get details from each issue, not only the first one.





New Bluetooth Low Energy OBD2 dongles

We are glad to announce that OBD Auto Doctor supports now four new Bluetooth Low Energy OBD2 dongles. These are Carista, Vgate, LELink and Viecar.

This is especially great news for iPhone users. Because, for technical reasons, iPhones don’t support traditional Bluetooth dongles at all. Now, the latest iPhone app has support for these four new dongles. The app supported already the Kiwi 3 before. iPhone users have now five different Bluetooth dongles to pick from.

Carista, Vgate and Viecar all have a chip containing support for both traditional Bluetooth as well as Bluetooth Low Energy (BLE). This is nice if you want to use the same dongle with your computer too. Not all computers have support for BLE but instead they have traditional Bluetooth. In this case, your computer too can communicate with the dongle. No need for hardware upgrade.

LELink is pure BLE device in similar way to Kiwi 3. It doesn’t have support for traditional Bluetooth.

Carista

Carista dongle is a reliable and safe adapter. The dongle has solid quality and small form factor.

Our iPhone app communicates with the dongle using Bluetooth Low Energy. Other OBD Auto Doctor software products use basic Bluetooth connection with it.

Carista dongle

Vgate

Vgate is a small and performing dongle. Vgate has several BLE products that we support. These include iCar Pro, iCar2 and iCar1 among others.

Remember to pick the Bluetooth Low Energy version if you need the iOS support. Other OBD Auto Doctor software uses the normal Bluetooth connection with it. Just like with Carista.

Carista dongle

LELink is pure Bluetooth Low Energy adapter.

At the time of publishing this blog post, OBD Auto Doctor has support for it only on iOS and Android. Support on other platforms might be added later.

LELink dongle

Viecar

Viecar has similar housing as Carista but the software stack is not the same.

The adapter works with all OBD Auto Doctor mobile apps. Depending on the platform either basic Bluetooth or Bluetooth Low Energy connection is used.

Viecar dongle

Where to get them?

We have listed these new and some other compatible dongles in the OBD Adapters & Dongles page. There you will find links to some stores that sell these dongles.


Editor's note: This post has been updated in April 2017: Added LELink and Viecar.