Praying sitting down with no excuse
Question

I work at a job where there are hundreds of people in the building. So to avoid performing my prayers late when I get home at night, I go to my car on my breaks and pray while sitting in the drivers seat. I tilt my head to perform Rukuh, and I bow my head further down to perform Sujud. I close my eyes to get Khushoo (concentration). Are my prayers valid?
Praise be to Allah.Standing during an obligatory prayer is one of the pillars or essential parts of the prayer, and it not permissible to omit it unless one is unable to stand because of sickness or intense fear, and the like. If a person omits it deliberately then his prayer is invalid. Hence it is clear to you that what you are doing by performing your prayers whilst sitting down is not permissible because you have no shar’i excuse, and your prayer sitting down is regarded as invalid. The evidence for that is the reports narrated by al-Bukhaari (1117) and others, from ‘Imraan ibn Husayn (may Allah be pleased with him) who said: “I had haemorrhoids and I asked the Prophet (peace and blessings of Allah be upon him) about prayer. He said: ‘Pray standing up. If you cannot, them pray sitting down, and if you cannot, then on your side.’” See al-Sharh al-Mumti’, 3/401. (For more details on the rulings on standing in prayer, see Question # 13340). 
Once this is clear to you, then you should note that it is also not permissible for you to delay prayers until after the time for them has passed. Rather you must perform each prayer at the time that is set for it in sharee’ah. Allah says (interpretation of the meaning): 
“Verily, As-Salaah (the prayer) is enjoined on the believers at fixed hours” [al-Nisa’ 4:103]
So you must strive to perform the prayers on time, standing up, in any suitable place, on the condition that the place be taahir (pure, clean). This is one of the things that sharee’ah makes easy, because Allah has given the ummah of Muhammad the unique privilege of permitting prayer in any clean place, as the Prophet (peace and blessings of Allah be upon him) said: “I have been given five things that were not given to anyone before me: I have been supported by fear (struck into the hearts of my enemies) for the distance of a month’s travel; the earth has been made a place of prayer and a means of purification for me, so that whenever the time for prayer comes, any man of my ummah can pray wherever he is; war-booty has been made permissible for me, when it was not permitted to anyone before me; I have been given the power of intercession; and Prophets (before me) were sent only to their own people, but I have been sent to all of mankind.” (Narrated by al-Bukhaari, 335, and others). 
If there is no mosque nearby where you can pray in congregation, then it is permissible for you to pray in any clean place, apart from a few places where Islam forbids praying, such as graveyards and land that has been seized unjustly by force. And Allah knows best.